Microorganisms and methods for producing 2-pyrone-4,6-dicarboxylic acid and other compounds

ABSTRACT

Recombinant microorganisms configured for enhanced production of compounds such as 2-pyrone-4,6-dicarboxylic acid (PDC) and methods of using the recombinant microorganisms for the production of these compounds. The recombinant microorganisms include one or more modifications that reduce 2-pyrone-4,6-dicarboxylic acid (PDC) hydrolase activity, 4-carboxy-2-hydroxy-6-methoxy-6-oxohexa-2,4-dienoate (CHMOD) cis-trans isomerase activity, 4-carboxy-2-hydroxy-6-methoxy-6-oxohexa-2,4-dienoate (CHMOD) methyl esterase activity, and/or vanillate/3-O-methylgallate O-demethylase activity. The recombinant microorganisms can be used to generate PDC from media comprising plant-derived phenolics, such as syringyl phenolics, guaiacyl phenolics, and p-hydroxyphenyl phenolics. The plant-derived phenolics can be derived from pretreated lignin, including depolymerized lignin or other chemically altered lignin.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under DE-FC02-07ER64494 and DE-SC0018409 awarded by the US Department of Energy. The government has certain rights in the invention.

SEQUENCE LISTING

The instant application contains a Sequence Listing which has been submitted in ASCII format via EFS-Web and is hereby incorporated by reference in its entirety. The ASCII copy, created on Feb. 14, 2020, is named USPTO-200219-Nonprovisional_Patent_Application-P180219US02-SEQ_LIST.txt and is 55,014 bytes in size.

FIELD OF THE INVENTION

The invention is directed to recombinant microorganisms configured for enhanced production of compounds such as 2-pyrone-4,6-dicarboxylic acid (PDC) and methods of using the recombinant microorganisms for the production of these compounds.

BACKGROUND

Plant biomass contains three main types of natural polymers: cellulose, hemicellulose, and lignin. Lignin is a heteropolymer of phenylpropanoids containing 4-hydroxy (H); 3-methoxyphenyl (G); and 3,5-dimethoxyphenyl (S) units in different proportions among plant species. Hardwoods contain between 18% and 25% lignin, with mostly G and S units. Softwoods contain between 25% and 35% lignin, with predominantly G and H units. Grasses contain between 10% and 30% lignin, with G, S, and H units present in similar proportions. Chemical lignin depolymerization treatments can produce diverse aromatic compounds that conserve the original H, G, and S units, with varying substitutions of the alkyl side chain.

This invention shows that 2-pyrone-4,6-dicarboxylic acid (PDC) is a metabolic intermediate of the biological catabolism of numerous lignocellulose-derived aromatic compounds containing H, G, and S units. PDC can be used as a natural building block and additive to plastic polymers and also shows a strong and unique binding capacity for certain metals.

Previous attempts to engineer bacterial strains to produce PDC have resulted in microorganisms able to convert selected single aromatic compounds or defined mixtures of a few compounds into PDC. The attempts have involved the addition of selected genes to bacterial strains that can allow aromatic compounds to pass through the cellular membrane but cannot naturally process them. Although these strategies have been effective in transforming selected aromatic compounds into PDC, no effective biological method is currently available to convert a wide range of biomass-derived aromatic compounds, including G, H, and S units plus others, into PDC.

Microorganisms and methods for converting a wide range of biomass-derived aromatic compounds into PDC are needed.

SUMMARY OF THE INVENTION

The invention is directed to recombinant microorganisms configured for enhanced production of compounds such as 2-pyrone-4,6-dicarboxylic acid (PDC) and methods of using the recombinant microorganisms for the production of these compounds.

The objects and advantages of the invention will appear more fully from the following detailed description of the preferred embodiment of the invention made in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

FIG. 1. Predicted pathways of S unit (syringic acid), G unit (vanillic acid), and H unit (p-hydroxybenzoic acid) metabolism in N. aromaticivorans DSM12444. In this model, deletions of the genes ligI (Saro_2819), desC (Saro_2864), and desD (Saro_2865) are hypothesized to enable the funneling (represented by light blue arrows) of S, G, and H lignocellulosic biomass-derived aromatic compounds into 2-pyrone-4,6-dicarboxylic acid (PDC). Abbreviations: 3-methylgallic acid, 3-MGA; 4-carboxy-2-hydroxy-6-methoxy-6-oxohexa-2,4-dienoate, CHMOD; 4-carboxy-2-hydroxy-cis,cis-muconate-6-semialdehyde, CHMS; 4-oxalomesaconate, OMA.

FIG. 2. Cell density of representative N. aromaticivorans cultures grown on 3 mM vanillic acid (panel A) or 3 mM p-hydroxybenzoic acid (panel B). Parent strain 12444Δ1879 represented by squares and dashed line; strain 12444ΔligI represented by circles.

FIG. 3. Cell density and extracellular metabolite concentration of representative N. aromaticivorans strain 12444ΔligI cultures grown on a combination of 3 mM vanillic acid and 3 mM glucose (panels A and C) or a combination of 3 mM p-hydroxybenzoic acid and 3 mM glucose (panels B and D).

FIG. 4. Cell density and extracellular metabolite concentrations of N. aromaticivorans strains 12444ΔligI (solid circles) or 12444ΔligIΔdesCD (solid triangles) grown on a combination of glucose and vanillin (A), p-hydroxybenzaldehyde (B), ferulic acid (C), p-coumaric acid (D), and syringaldehyde (E).

FIG. 5. Cell density and extracellular metabolite concentrations of representative cultures of N. aromaticivorans strains 12444Δ1879 (panels A and C) and 12444ΔligI (panels B and D) grown in media containing syringic acid.

FIG. 6. Cell densities and extracellular metabolite concentrations of N. aromaticivorans strains 12444ΔdesCD (left-hand side panels) and 12444ΔligIΔdesCD (right-hand side panels) grown on 3 mM syringic acid (panels A and B) or a combination of 3 mM syringic acid and 3 mM glucose (panels C to F).

FIG. 7. Cell density and extracellular metabolite concentrations of representative N. aromaticivorans strain 12444ΔdesCD cultured on 3 mM vanillic acid.

FIG. 8. GC-MS peaks of compounds identified in media containing glucose plus the products of formic-acid-induced depolymerization of oxidized poplar lignin; before inoculation (A), after growth of N. aromaticivorans strain 12444ΔligIΔdesCD (B), after growth of N. aromaticivorans strain 12444Δ1879 (C). Only strain 12444ΔligIΔdesCD accumulates PDC in the growth medium. Panel D shows the absence of additional peaks in an abiotic control experiment.

FIG. 9. Cell density (panels A and B) and extracellular metabolite concentrations (panels C to F) of representative cultures of N. aromaticivorans strains 12444ΔligIΔdesCD (left-hand side panels) and 12444Δ1879 (right-hand side panels) grown on formic-acid-induced depolymerized poplar lignin supplemented with glucose. Panels C and D show extracellular concentrations of lignin-derived aromatic compounds and PDC as a product, and panels E and F show extracellular concentrations of glucose and formic acid. Formic acid is present in the low molecular weight products of chemical depolymerization, whereas glucose was added to enhance bacterial cell growth.

FIGS. 10A-10D. GPC chromatograms of media containing glucose plus the products of formic-acid-induced depolymerization of oxidized poplar lignin; before inoculation (FIG. 10A), abiotic control after 78 hours of incubation (FIG. 10B), after growth of N. aromaticivorans strain 12444Δ1879 (FIG. 10C), after growth of N. aromaticivorans strain 12444ΔligIΔdesCD (FIG. 10D).

FIG. 11. GPC chromatogram of the “oligomers” range at λ=254 of media containing glucose plus the products of formic-acid-induced depolymerization of oxidized poplar lignin. Mw: weight average molecular weight; Mn: number average molecular weight; Mw/Mn: dispersity index.

FIG. 12. Cell density and extracellular metabolite concentrations of representative N. aromaticivorans strain 12444ΔligIΔdesCD cultures grown on minimal media supplemented with S-diketone and glucose (panels A and C) or G-diketone and glucose (panels B and D).

FIG. 13. Extracellular metabolite concentrations of a N. aromaticivorans strain 12444ΔligIΔdesCD culture fed with a concentrated mixture of vanillic acid, vanillin, and glucose. A maximum PDC concentration of 26.7 mM was observed after 48 hours of cultivation.

FIGS. 14A and 14B. Production of PDC from mild alkaline-pretreated poplar (FIG. 14A) and sorghum (FIG. 14B) lignocellulosic biomass with the N. aromaticivorans 12444ΔligIΔdesCD strain.

FIGS. 15A-15C. Production of PDC from isolated compounds found in depolymerized lignocellulosic biomass pretreated with γ-valerolactone (GVL) and subjected to hydrogenolysis, using the N. aromaticivorans 12444ΔligIΔdesCD strain.

FIGS. 16A-16D. Production of PDC from poplar (FIG. 16A), switchgrass (FIG. 16B), sorghum (FIG. 16C), and maple (FIG. 16D) lignin isolated from lignocellulosic biomass pretreated with γ-valerolactone (GVL) and subjected to hydrogenolysis, using the N. aromaticivorans 12444ΔligIΔdesCD strain. Triplicate experiments with each biomass substrate are shown.

FIGS. 17A-17H. Cell growth and production of PDC from vanillic acid (“VA”) with the N. aromaticivorans 12444ΔligIΔdesCD strain (labeled as the “PDC” strain) (FIG. 17A) and variants comprising additional deletions (FIGS. 17B-17H). The additional deletions include ΔligM (ΔSaro_2861) (FIG. 17B), ΔdesA (ΔSaro_2404) (FIG. 17C), ΔvanA (ΔSaro_1872) (FIG. 17D), ΔligMΔdesA (ΔSaro_2861ΔSaro_2404) (FIG. 17E), ΔligMΔvanA (ΔSaro_2861ΔSaro_1872) (FIG. 17F), ΔdesAΔvanA (ΔSaro_2404ΔSaro_1872) (FIG. 17G), and ΔligMΔdesAΔvanA (ΔSaro_2861ΔSaro_2404ΔSaro_1872) (FIG. 17H).

FIGS. 18A-18H. Cell growth and production of PDC from syringic acid (“SA”) with the N. aromaticivorans 12444ΔligIΔdesCD strain (labeled as the “PDC” strain) (FIG. 18A) and variants comprising additional deletions (FIGS. 18B-18H). The additional deletions include ΔligM (ΔSaro_2861) (FIG. 18B), ΔdesA (ΔSaro_2404) (FIG. 18C), ΔvanA (ΔSaro_1872) (FIG. 18D), ΔligMΔdesA (ΔSaro_2861ΔSaro_2404) (FIG. 18E), ΔligMΔvanA (ΔSaro_2861ΔSaro_1872) (FIG. 18F), ΔdesAΔvanA (ΔSaro_2404ΔSaro_1872) (FIG. 18G), and ΔligMΔdesAΔvanA (ΔSaro_2861ΔSaro_2404ΔSaro_1872) (FIG. 18H).

FIG. 19. Predicted pathways in addition to those shown in FIG. 1 of S unit (syringic acid), G unit (vanillic acid), and H unit (p-hydroxybenzoic acid) metabolism in N. aromaticivorans DSM12444.

DETAILED DESCRIPTION OF THE INVENTION

One aspect of the invention is directed to recombinant microorganisms. The recombinant microorganisms of the invention can be configured for enhanced production of 2-pyrone-4,6-dicarboxylic acid (PDC) or other compounds. The recombinant microorganisms of the invention comprise one or more modifications that reduce the activity of one or more genes or gene products, increase the activity of one or more genes or gene products, or reduce the activity of one or more genes or gene products and increase the activity of one or more genes or gene products. The recombinant microorganisms with the modifications can exhibit enhanced production or 2-pyrone-4,6-dicarboxylic acid (PDC) or other compounds with respect to corresponding microorganisms not comprising the modifications.

“Modifications that reduce the activity of one or more genes or gene products” refers to any modification to a microorganism that decreases or ablates expression of the gene and thus production of the gene product and/or decreases or ablates the functioning of the gene product per se. Decreasing or ablating the functioning of a gene product may comprise decreasing or ablating the specific activity of a gene product. Exemplary modifications that reduce the activity of one or more genes or gene products include genetic modifications. The genetic modifications include mutations to a gene that decrease or ablate expression of the gene in producing the gene product. Such mutations may include mutations to the coding sequence, the promoter, an enhancer, or any other part of the gene. The genetic mutations also include mutations to the coding sequence of a gene that decrease or ablate the functioning of a gene product expressed from the gene. The genetic mutations also include recombinant nucleotide sequences configured to express antisense RNAs or other molecules that decrease or ablate production of a gene product. The genetic modifications also include mutations to a first gene (such as a transcription factor or an inhibitor of a transcription factor) that affects the expression of a second gene. Other genetic modifications are described elsewhere herein. Unless explicitly stated otherwise or indicated from the context, reference to a modification that reduces the activity of any named gene (e.g., “LigI,” “Saro_1879”) or homolog thereof encompasses any modification that decreases the activity (e.g., expression and/or functionality) of the gene or homolog thereof and/or the gene product of the gene or homolog thereof, as described above and elsewhere herein.

“Modifications that increase the activity of one or more genes or gene products” refers to any modification to microorganism that increases expression of a gene in producing its gene product or increases the functioning of the gene product. “Increase” in this context refers to increasing beyond a positive baseline activity or increasing beyond null activity and thereby introducing a new activity. Exemplary modifications that increase the activity of one or more genes or gene products include genetic modifications. The genetic modifications include genetic modifications to a gene in a manner that increases expression of the gene in producing the gene product. Such modifications include operationally connecting the coding sequence to a stronger promoter or enhancer, etc., and/or introducing additional copies of the gene (whether the native gene or a recombinant version). The genetic modifications also include mutations to a first gene (such as a transcription factor or an inhibitor of a transcription factor) that affects the expression of a second gene. The genetic modifications also include one or more copies of a gene introduced into the microorganism. Other genetic modifications are described herein. Any modifications described herein can comprise recombinant genes. Unless explicitly stated otherwise or indicated from the context, reference to a modification that increases the activity of any named gene (e.g., “ligC,” “Saro_2811”) or homolog thereof encompasses any modification that increases the activity (e.g., expression and/or functionality) of the gene or homolog thereof and/or the gene product of the gene or homolog thereof, as described above and elsewhere herein.

“Corresponding microorganism” refers to a microorganism of the same species having the same or substantially same genetic and proteomic composition as a recombinant microorganism of the invention, with the exception of genetic and proteomic differences resulting from the modifications described herein for the recombinant microorganisms of the invention. In some versions, the corresponding microorganism is the native version of the recombinant microorganism of the invention, i.e., the unmodified microorganism as found in nature. The terms “microorganism” and “microbe” are used interchangeably herein.

In some versions, the recombinant microorganisms comprise one or more modifications with respect to a corresponding microorganism not comprising the one or more modifications. The one or more modifications can comprise a modification selected from the group consisting of a modification that reduces 2-pyrone-4,6-dicarboxylic acid (PDC) hydrolase activity with respect to the corresponding microorganism, a modification that reduces 4-carboxy-2-hydroxy-6-methoxy-6-oxohexa-2,4-dienoate (CHMOD) cis-trans isomerase activity with respect to the corresponding microorganism, a modification that reduces CHMOD methyl esterase activity with respect to the corresponding microorganism, a modification that reduces vanillate/3-O-methylgallate O-demethylase activity with respect to the corresponding microorganism, a modification that increases syringic acid O-demethylase activity with respect to the corresponding microorganism, a modification that increases aromatic 4,5 dioxygenase activity with respect to the corresponding microorganism, a modification that increases p-hydroxybenzoic acid 3-monooxygenase activity with respect to the corresponding microorganism, and a modification that increases 4-carboxy-2-ydroxy-cis,cis-muconate-6-semialdehyde (CHMS) dehydrogenase activity with respect to the corresponding microorganism.

PDC hydrolase activity comprises the ability to hydrolyze PDC to produce 4-oxalomesaconate (OMA). An exemplary PDC hydrolase is ligI/LigI (Saro_2819) of Novosphingobium aromaticivorans, the nucleic acid coding sequence of which is SEQ ID NO:1 and the protein sequence of which is SEQ ID NO:2. Homologs of the ligI/LigI of Novosphingobium aromaticivorans are known in other organisms. In some versions of the invention, the one or more modifications in the recombinant microorganisms can comprise a genetic modification that reduces PDC hydrolase activity. A genetic modification that reduces PDC hydrolase activity can comprise a genetic modification to a PDC hydrolase gene. A genetic modification to a PDC hydrolase gene can comprise a substitution or insertion in or a complete or partial deletion of the PDC hydrolase gene. In some versions, the one or more modifications in the recombinant microorganisms can comprise a modification that reduces activity of ligI or a homolog thereof. A modification that reduces activity of ligI or a homolog thereof can comprise a genetic modification of ligI or a homolog thereof. A genetic modification of ligI or a homolog thereof can comprise a substitution or insertion in or a complete or partial deletion of ligI or a homolog thereof.

CHMOD cis-trans isomerase activity comprises the ability to isomerize stereoisomers of CHMOD. An exemplary CHMOD cis-trans isomerase is desD/DesD (Saro_2865) of Novosphingobium aromaticivorans, the nucleic acid coding sequence of which is SEQ ID NO:3 and the protein sequence of which is SEQ ID NO:4. Homologs of the desD/DesD of Novosphingobium aromaticivorans are known in other organisms. In some versions of the invention, the one or more modifications in the recombinant microorganisms can comprise a genetic modification that reduces CHMOD cis-trans isomerase activity. A genetic modification that reduces CHMOD cis-trans isomerase activity can comprise a genetic modification to a CHMOD cis-trans isomerase gene. A genetic modification to a CHMOD cis-trans isomerase gene can comprise a substitution or insertion in or a complete or partial deletion of the CHMOD cis-trans isomerase gene. In some versions, the one or more modifications in the recombinant microorganisms can comprise a modification that reduces activity of desD or a homolog thereof. A modification that reduces activity of desD or a homolog thereof can comprise a genetic modification of desD or a homolog thereof. A genetic modification of desD or a homolog thereof can comprise a substitution or insertion in or a complete or partial deletion of desD or a homolog thereof.

CHMOD methyl esterase activity comprises the ability to demethylate CHMOD to produce OMA. An exemplary CHMOD methyl esterase is desC/DesC (Saro_2864) of Novosphingobium aromaticivorans, the nucleic acid coding sequence of which is SEQ ID NO:5 and the protein sequence of which is SEQ ID NO:6. Homologs of the desC/DesC of Novosphingobium aromaticivorans are known in other organisms. In some versions of the invention, the one or more modifications in the recombinant microorganisms can comprise a genetic modification that reduces CHMOD methyl esterase activity. A genetic modification that reduces CHMOD methyl esterase activity can comprise a genetic modification to a CHMOD methyl esterase gene. A genetic modification to a CHMOD methyl esterase gene can comprise a substitution or insertion in or a complete or partial deletion of the CHMOD methyl esterase gene. In some versions, the one or more modifications in the recombinant microorganisms can comprise a modification that reduces activity of desC or a homolog thereof. A modification that reduces activity of desC or a homolog thereof can comprise a genetic modification of desC or a homolog thereof. A genetic modification of desC or a homolog thereof can comprise a substitution or insertion in or a complete or partial deletion of desC or a homolog thereof.

Vanillate/3-O-methylgallate O-demethylase activity comprises the ability to O-demethylate substrates such as vanillate and/or 3-methoxygallic acid. Vanillate/3-O-methylgallate O-demethylases include enzymes having activity characterized under one more of Enzyme Commission (EC) Numbers 2.1.1.341 and 1.14.13.82. An exemplary vanillate/3-O-methylgallate O-demethylase is ligM/LigM (Saro_2861) of Novosphingobium aromaticivorans, the nucleic acid coding sequence of which is SEQ ID NO:7 and the protein sequence of which is SEQ ID NO:8. Homologs of the ligM/LigM of Novosphingobium aromaticivorans are known in other organisms. Another exemplary vanillate/3-O-methylgallate O-demethylase is vanA/VanA (Saro_1872) of Novosphingobium aromaticivorans, the nucleic acid coding sequence of which is SEQ ID NO:9 and the protein sequence of which is SEQ ID NO:10. Homologs of the vanA/VanA of Novosphingobium aromaticivorans are known in other organisms. In some versions of the invention, the one or more modifications in the recombinant microorganisms can comprise a genetic modification that reduces vanillate/3-O-methylgallate O-demethylase activity. A genetic modification that reduces vanillate/3-O-methylgallate O-demethylase activity can comprise a genetic modification to a vanillate/3-O-methylgallate O-demethylase gene. A genetic modification to a vanillate/3-O-methylgallate O-demethylase gene can comprise a substitution or insertion in or a complete or partial deletion of the vanillate/3-O-methylgallate O-demethylase gene. In some versions, the one or more modifications in the recombinant microorganisms can comprise a modification that reduces activity of ligM, a homolog of ligM, vanA, a homolog of vanA, or a combination thereof. A modification that reduces activity of ligM, vanA, or homologs thereof can comprise a genetic modification of ligM, vanA, or homologs thereof. A genetic modification of ligM, vanA, or homologs can comprise a substitution or insertion in or a complete or partial deletion of ligM, vanA, or homologs thereof. Vanillate/3-O-methylgallate O-demethylases such as LigM can also react with 3-methylgallate as a substrate, and also likely with syringic acid. Vanillate/3-O-methylgallate O-demethylases such as VanA can also likely react with 3-methylgallate as a substrate.

Syringic acid O-demethylase activity comprises the ability to demethylate syringic acid to produce 3-methylgallate (3-MGA). An exemplary syringic acid O-demethylase is desA/DesA (Saro_2404) of Novosphingobium aromaticivorans, the nucleic acid coding sequence of which is SEQ ID NO:11 and the protein sequence of which is SEQ ID NO:12. Homologs of the desA/DesA of Novosphingobium aromaticivorans are known in other organisms. In some versions of the invention, the one or more modifications in the recombinant microorganisms can comprise a genetic modification that increases syringic acid O-demethylase activity. A genetic modification that increases syringic acid O-demethylase activity can comprise an introduction of a recombinant (genetically modified) syringic acid O-demethylase gene. The introduction of the recombinant syringic acid O-demethylase gene can occur by newly introducing a recombinant syringic acid O-demethylase gene to the microorganism or modifying a syringic acid O-demethylase gene already present in the microorganism. In some versions, the one or more modifications in the recombinant microorganisms can comprise a modification that increases activity of desA or a homolog thereof. A modification that increases activity of desA or a homolog thereof can comprise an introduction of a recombinant desA gene or a homolog thereof. The introduction of the recombinant desA gene or homolog thereof can occur by newly introducing a recombinant desA gene or homolog thereof to the microorganism or modifying a desA gene or homolog thereof already present in the microorganism. Syringic acid O-demethylases such as DesA may also react with vanillic acid as a substrate.

Aromatic 4,5 dioxygenase activity comprises the ability to catalyze ring opening of aromatic compounds such as 3-MGA and protocatechuic acid to produce compounds such as CHMOD and CHMS. An exemplary aromatic 4,5 dioxygenase is ligAB/LigAB (Saro_2813/2812; Saro_1233/1234) of Novosphingobium aromaticivorans. The nucleic acid coding sequence of the Saro_2813 ligA gene is SEQ ID NO:13, and the amino acid sequence of the Saro_2813 LigA subunit is SEQ ID NO:14. The nucleic acid coding sequence of the Saro_2812 ligB gene is SEQ ID NO:15, and the amino acid sequence of the Saro_2812 LigB subunit is SEQ ID NO:16. The nucleic acid coding sequence of the Saro_1233 ligA gene is SEQ ID NO:17, and the amino acid sequence of the Saro_1233 LigA subunit is SEQ ID NO:18. The nucleic acid coding sequence of the Saro_1234 ligB gene is SEQ ID NO:19, and the amino acid sequence of the Saro_1234 LigB subunit is SEQ ID NO:20. Homologs of the ligAB/LigAB of Novosphingobium aromaticivorans are known in other organisms. In some versions of the invention, the one or more modifications in the recombinant microorganisms can comprise a genetic modification that increases aromatic 4,5 dioxygenase activity. A genetic modification that increases aromatic 4,5 dioxygenase activity can comprise an introduction of a recombinant (genetically modified) aromatic 4,5 dioxygenase gene. The introduction of the recombinant aromatic 4,5 dioxygenase gene can occur by newly introducing a recombinant aromatic 4,5 dioxygenase gene to the microorganism or modifying an aromatic 4,5 dioxygenase gene already present in the microorganism. In some versions, the one or more modifications in the recombinant microorganisms can comprise a modification that increases activity of ligAB or a homolog thereof. A modification that increases activity of ligAB or a homolog thereof can comprise an introduction of recombinant ligAB genes or homologs thereof. The introduction of the recombinant ligAB genes or homologs thereof can occur by newly introducing recombinant ligAB genes or homologs thereof to the microorganism or modifying ligAB genes or homologs thereof already present in the microorganism.

p-Hydroxybenzoic acid 3-monooxygenase activity comprises the ability to hydroxylate p-hydroxybenzoic acid. An exemplary p-hydroxybenzoic acid 3-monooxygenase is Saro_2436 of Novosphingobium aromaticivorans, the nucleic acid coding sequence of which is SEQ ID NO:21 and the protein sequence of which is SEQ ID NO:22. Homologs of Saro_2436 of Novosphingobium aromaticivorans are known in other organisms. In some versions of the invention, the one or more modifications in the recombinant microorganisms can comprise a genetic modification that increases p-hydroxybenzoic acid 3-monooxygenase activity. A genetic modification that increases p-hydroxybenzoic acid 3-monooxygenase activity can comprise an introduction of a recombinant (genetically modified) p-hydroxybenzoic acid 3-monooxygenase gene. The introduction of the recombinant p-hydroxybenzoic acid 3-monooxygenase gene can occur by newly introducing a recombinant p-hydroxybenzoic acid 3-monooxygenase gene to the microorganism or modifying a p-hydroxybenzoic acid 3-monooxygenase gene already present in the microorganism. In some versions, the one or more modifications in the recombinant microorganisms can comprise a modification that increases activity of Saro_2436 or a homolog thereof. A modification that increases activity of Saro_2436 or a homolog thereof can comprise an introduction of a recombinant Saro_2436 gene or a homolog thereof. The introduction of the recombinant Saro_2436 gene or homolog thereof can occur by newly introducing a recombinant Saro_2436 gene or homolog thereof to the microorganism or modifying a Saro_2436 gene or homolog thereof already present in the microorganism.

CHMS dehydrogenase activity comprises the ability to oxidize CHMS to produce PDC. An exemplary CHMS dehydrogenase is ligC/LigC (Saro_2811) of Novosphingobium aromaticivorans, the nucleic acid coding sequence of which is SEQ ID NO:23 and the protein sequence of which is SEQ ID NO:24. Homologs of ligC/LigC of Novosphingobium aromaticivorans are known in other organisms. In some versions of the invention, the one or more modifications in the recombinant microorganisms can comprise a genetic modification that increases CHMS dehydrogenase activity. A genetic modification that increases CHMS dehydrogenase activity can comprise an introduction of a recombinant (genetically modified) CHMS dehydrogenase gene. The introduction of the recombinant CHMS dehydrogenase gene can occur by newly introducing a recombinant CHMS dehydrogenase gene to the microorganism or modifying a CHMS dehydrogenase gene already present in the microorganism. In some versions, the one or more modifications in the recombinant microorganisms can comprise a modification that increases activity of ligC or a homolog thereof. A modification that increases activity of ligC or a homolog thereof can comprise an introduction of a recombinant ligC gene or a homolog thereof. The introduction of the recombinant ligC gene or homolog thereof can occur by newly introducing a recombinant ligC gene or homolog thereof to the microorganism or modifying a ligC gene or homolog thereof already present in the microorganism.

The recombinant microorganisms in preferred versions of the invention are configured to exhibit enhanced production of PDC with respect to a corresponding microorganism. The recombinant microorganisms in such versions may include any one or more of the modifications described herein. Preferred modifications that confer enhancement of PDC production include modifications that reduce PDC hydrolase activity, CHMOD cis-trans isomerase activity, CHMOD methyl esterase activity, and/or vanillate/3-O-methylgallate O-demethylase activity. The additional modifications described herein can be implemented by themselves or in combination with the modifications that reduce PDC hydrolase activity, CHMOD cis-trans isomerase activity, CHMOD methyl esterase activity, and/or vanillate/3-O-methylgallate O-demethylase activity.

Modifications that reduce the activity of a gene or gene product includes any modification to a microorganism that ablates, reduces, inhibits, or otherwise disrupts production of a gene product, renders a produced gene product non-functional, or otherwise reduces or ablates a produced gene product's activity. Accordingly, in some instances, production of a gene product may be completely shut down. “Gene product” refers to products such as an mRNA or a polypeptide encoded and produced by a particular gene. “Gene” refers to a nucleic acid sequence capable of producing a gene product and may include such genetic elements as a coding sequence together with any other genetic elements required for transcription and/or translation of the coding sequence. Such genetic elements may include a promoter, an enhancer, and/or a ribosome binding site (RBS), among others.

There are many well-known ways to reduce the activity of a gene or gene product. This can be accomplished, for example, by introducing one or more genetic modifications. As used herein, “genetic modifications” refer to any differences in the nucleic acid composition of a cell, whether in the cell's native chromosome or in endogenous or exogenous non-chromosomal plasmids harbored within the cell. Examples of genetic modifications that reduce the activity of a gene or gene product include but are not limited to substitutions, partial or complete deletions, insertions, or other variations to a coding sequence or a sequence controlling the transcription or translation of a coding sequence, such as placing a coding sequence under the control of a less active promoter, etc. In some versions, a gene or coding sequence can be replaced with a selection marker or screenable marker. In some versions, the genetic modifications can include the introduction of constructs that express ribozymes or antisense sequences that target the mRNA of the gene of interest. Various other genetic modifications that reduce the activity of a gene or gene product are described elsewhere herein. Various methods for introducing genetic modifications are well known in the art and include homologous recombination, among other mechanisms. See, e.g., Green et al., Molecular Cloning: A laboratory manual, 4^(th) ed., Cold Spring Harbor Laboratory Press (2012) and Sambrook et al., Molecular Cloning: A Laboratory Manual, 3^(rd) ed., Cold Spring Harbor Laboratory Press (2001). In some instances, reducing the activity of a gene or gene product can be accomplished by chemically inhibiting the activity of a gene product with a small-molecule inhibitor, by expressing a protein that interferes with the activity of the gene product, or by other means.

“Increasing expression” or grammatical variants thereof may refer to expressing a gene product not made by the corresponding microorganism or expressing more of a gene product already made by the corresponding microorganism. Modifying the recombinant microorganisms to increase expression of the gene products described herein can be performed using any methods currently known in the art or discovered in the future. Examples include genetically modifying the microorganism and culturing the microorganism in the presence of factors that increase expression of the gene product. Suitable methods for genetic modification include but are not limited to placing the coding sequence under the control of a more active promoter, increasing the copy number of genes comprising the coding sequence, introducing a translational enhancer on a gene comprising the coding sequence (see, e.g., Olins et al. Journal of Biological Chemistry, 1989, 264(29):16973-16976), and/or modifying factors (e.g., transcription factors or genes therefor) that control expression of a gene comprising the coding sequence. Increasing the copy number of genes comprising a coding sequence can be performed by introducing one or more additional copies of the native gene to the microorganism, introducing one or more a heterologous homologs to the microorganism, introducing one or more copies of recombinant versions of the native gene or heterologous homolog to the microorganism, etc. Genes expressing a given coding sequence may be incorporated into the microbial genome or included on an extrachromosomal genetic construct such as a plasmid. “Exogenous” used in reference to a genetic element means the genetic element is a non-native genetic element. “Heterologous” used in reference to a genetic element means that the genetic element is derived from a different species. A genetic element, such as a promoter, that controls or affects the activity of another genetic element, such as a coding sequence, is herein described as being “operationally connected” thereto.

Some of the microorganisms of the invention may include at least one recombinant nucleic acid configured to express or overexpress a particular product. “Recombinant” as used herein with reference to a nucleic acid molecule or polypeptide is one that has a sequence that is not naturally occurring. The recombinant nucleic acid molecule or polypeptide can be made, for example, by chemical synthesis or by the artificial manipulation of isolated segments of nucleic acid molecules or polypeptides using genetic engineering techniques. A recombinant cell or microorganism is one that contains a recombinant nucleic acid molecule or polypeptide. “Overexpress” as used herein means that a particular gene product is produced at a higher level in one cell, such as a recombinant cell, than in a corresponding cell. For example, a microorganism that includes a recombinant nucleic acid configured to overexpress a gene product produces the gene product at a greater amount than a microorganism of the same species that does not include the recombinant nucleic acid.

In general, proteins and/or protein sequences are “homologous” when they are derived, naturally or artificially, from a common ancestral protein or protein sequence. Similarly, nucleic acids and/or nucleic acid sequences are homologous when they are derived, naturally or artificially, from a common ancestral nucleic acid or nucleic acid sequence. Homology is generally inferred from sequence similarity between two or more nucleic acids or proteins (or sequences thereof). The precise percentage of similarity between sequences that is useful in establishing homology varies with the nucleic acid and protein at issue, but as little as 25% sequence similarity (e.g., identity) over 50, 100, 150 or more residues (nucleotides or amino acids) is routinely used to establish homology (e.g., over the full length of the two sequences to be compared). Higher levels of sequence similarity (e.g., identity), e.g., 30%, 35% 40%, 45% 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 99% or more, can also be used to establish homology. Accordingly, homologous sequences of the sequences described herein include coding sequences, genes, or gene products (e.g., proteins), respectively, having at least about 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 99% identity to the sequences described herein. Methods for determining sequence similarity percentages (e.g., BLASTP and BLASTN using default parameters) are described herein and are generally available. The homologous proteins should demonstrate comparable activities and, if an enzyme, participate in the same or analogous pathways. “Orthologs” are genes or coding sequences thereof in different species that evolved from a common ancestral gene by speciation. Normally, orthologs retain the same or similar function in the course of evolution. As used herein “orthologs” are included in the term “homologs.” Homologs also include sequences at least 90%, 95%, or 97% or more identical to the orthologs.

For sequence comparison and homology determination, one sequence typically acts as a reference sequence to which test sequences are compared. When using a sequence comparison algorithm, test and reference sequences are input into a computer, subsequence coordinates are designated, if necessary, and sequence algorithm program parameters are designated. The sequence comparison algorithm then calculates the percent sequence identity for the test sequence(s) relative to the reference sequence based on the designated program parameters. A typical reference sequence of the invention is any nucleic acid or amino acid sequence described herein.

Optimal alignment of sequences for comparison can be conducted, e.g., by the local homology algorithm of Smith & Waterman, Adv. Appl. Math. 2:482 (1981), by the homology alignment algorithm of Needleman & Wunsch, J. Mol. Biol. 48:443 (1970), by the search for similarity method of Pearson & Lipman, Proc. Nat'l. Acad. Sci. USA 85:2444 (1988), by computerized implementations of these algorithms (GAP, BESTFIT, FASTA, and TFASTA in the Wisconsin Genetics Software Package, Genetics Computer Group, 575 Science Dr., Madison, Wis.), or by visual inspection (see Current Protocols in Molecular Biology, F. M. Ausubel et al., eds., Current Protocols, a joint venture between Greene Publishing Associates, Inc. and John Wiley & Sons, Inc., (supplemented through 2008)).

One example of an algorithm that is suitable for determining percent sequence identity and sequence similarity for purposes of defining homologs is the BLAST algorithm, which is described in Altschul et al., J. Mol. Biol. 215:403-410 (1990). Software for performing BLAST analyses is publicly available through the National Center for Biotechnology Information. This algorithm involves first identifying high scoring sequence pairs (HSPs) by identifying short words of length W in the query sequence, which either match or satisfy some positive-valued threshold score T when aligned with a word of the same length in a database sequence. T is referred to as the neighborhood word score threshold (Altschul et al., supra). These initial neighborhood word hits act as seeds for initiating searches to find longer HSPs containing them. The word hits are then extended in both directions along each sequence for as far as the cumulative alignment score can be increased. Cumulative scores are calculated using, for nucleotide sequences, the parameters M (reward score for a pair of matching residues; always>0) and N (penalty score for mismatching residues; always<0). For amino acid sequences, a scoring matrix is used to calculate the cumulative score. Extension of the word hits in each direction are halted when: the cumulative alignment score falls off by the quantity X from its maximum achieved value; the cumulative score goes to zero or below, due to the accumulation of one or more negative-scoring residue alignments; or the end of either sequence is reached. The BLAST algorithm parameters W, T, and X determine the sensitivity and speed of the alignment. The BLASTN program (for nucleotide sequences) uses as defaults a wordlength (W) of 11, an expectation (E) of 10, a cutoff of 100, M=5, N=−4, and a comparison of both strands. For amino acid sequences, the BLASTP program uses as defaults a wordlength (W) of 3, an expectation (E) of 10, and the BLOSUM62 scoring matrix (see Henikoff & Henikoff (1989) Proc. Natl. Acad. Sci. USA 89:10915).

In addition to calculating percent sequence identity, the BLAST algorithm also performs a statistical analysis of the similarity between two sequences (see, e.g., Karlin & Altschul, Proc. Natl. Acad. Sci. USA 90:5873-5787 (1993)). One measure of similarity provided by the BLAST algorithm is the smallest sum probability (P(N)), which provides an indication of the probability by which a match between two nucleotide or amino acid sequences would occur by chance. For example, a nucleic acid is considered similar to a reference sequence if the smallest sum probability in a comparison of the test nucleic acid to the reference nucleic acid is less than about 0.1, more preferably less than about 0.01, and most preferably less than about 0.001. The above-described techniques are useful in identifying homologous sequences for use in the methods described herein.

The terms “identical” or “percent identity”, in the context of two or more nucleic acid or polypeptide sequences, refer to two or more sequences or subsequences that are the same or have a specified percentage of amino acid residues or nucleotides that are the same, when compared and aligned for maximum correspondence, as measured using one of the sequence comparison algorithms described above (or other algorithms available to persons of skill) or by visual inspection.

The phrase “substantially identical”, in the context of two nucleic acids or polypeptides refers to two or more sequences or subsequences that have at least about 60%, about 65%, about 70%, about 75%, about 80%, about 85%, about 90, about 95%, about 98%, or about 99% or more nucleotide or amino acid residue identity, when compared and aligned for maximum correspondence, as measured using a sequence comparison algorithm or by visual inspection. Such “substantially identical” sequences are typically considered to be “homologous” without reference to actual ancestry. Preferably, the “substantial identity” exists over a region of the sequences that is at least about 50 residues in length, more preferably over a region of at least about 100 residues, and most preferably, the sequences are substantially identical over at least about 150 residues, at least about 250 residues, or over the full length of the two sequences to be compared.

Exogenous, heterologous nucleic acids encoding products to be expressed in the microorganism are preferably codon-optimized for the particular microorganism in which they are introduced. Codon optimization can be performed for any nucleic acid by a number of programs, including “GENEGPS”-brand expression optimization algorithm by DNA 2.0 (Menlo Park, Calif.), “GENEOPTIMIZER”-brand gene optimization software by Life Technologies (Grand Island, N.Y.), and “OPTIMUMGENE”-brand gene design system by GenScript (Piscataway, N.J.). Other codon optimization programs or services are well known and commercially available.

The recombinant microorganisms of the invention may comprise any type of microorganism. The microorganism may be prokaryotic or eukaryotic. Suitable prokaryotes include bacteria and archaea. Suitable types of bacteria include α- and γ-proteobacteria, gram-positive bacteria, gram-negative bacteria, ungrouped bacteria, phototrophs, lithotrophs, and organotrophs. Suitable eukaryotes include yeast and other fungi. The microorganism in some versions can be from an order selected from the group consisting of Sphingomonadales and Pseudomonadales. The microorganism in some versions can be from a family selected from the group consisting of Sphingomonadaceae and Pseudomonadaceae. The microorganism in some versions can be from a genus selected from the group consisting of Sphingomonas, Sphingobium, Sphingosinicella, Sphingopyxis, Novosphingobium, Pseudomonas, Erythrobacter (e.g., sp. SG61-1L), and Altererythrobacter. An exemplary microorganism from the genus Novosphingobium is Novosphingobium aromaticivorans. Novosphingobium aromaticivorans DSM12444 can naturally catabolize multiple aromatic compounds containing H, G, and S units via protocatechuic acid and 3-O-methylgallic acid as central metabolites, with PDC as a common intermediate.

The recombinant microorganisms of the invention preferably exhibit enhanced PDC production with respect to the corresponding microorganism when the recombinant microorganism and the corresponding organism are grown under identical conditions. The PDC production may be enhanced by a factor of at least about 1.1, at least about 1.5, at least about 2, at least about 2.5, at least about 3, at least about 3.5, at least about 4, at least about 4.5, at least about 5, at least about 5.5, at least about 6, or at least about 6.5 and/or up to about 6.5, up to about 7, or more. Such increases may reflect an increase by mass.

The recombinant microorganisms of the invention preferably exhibit enhanced PDC yield from certain substrates with respect to the corresponding microorganism when the recombinant microorganism and the corresponding organism are grown under identical conditions. The substrates may include one or more of vanillic acid, syringic acid, p-hydroxybenzoic acid, methyl guaiacol, propyl guaiacol, dihydroconiferyl alcohol, methyl syringol, p-hydroxy benzoic acid methyl ester, dihydrop-hydroxy cinnamic acid methyl ester, dihydrosyringol alcohol, and dihydroferulic acid methyl ester, among others. The PDC yield may be enhanced by a factor of at least about 1.1, at least about 1.5, at least about 2, at least about 2.5, at least about 3, at least about 3.5, at least about 4, at least about 4.5, at least about 5, at least about 5.5, at least about 6, or at least about 6.5 and/or up to about 6.5, up to about 7, or more. Such yields are determined on a mass basis.

The recombinant microorganisms of the invention preferably exhibit a yield from vanillic acid, syringic acid, or each of vanillic acid and syringic acid of at least about 50%, at least about 55%, at least about 60%, at least about 65%, at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, or at least about 98%, or at least about 99%. Such yields are determined on a mass basis.

The PDC can be produced by culturing a recombinant microorganism of the invention in a medium comprising a plant-derived phenolic. The plant-derived phenolic can comprise any of a number of phenolics obtained from processing plant lignocellulosic biomass. Exemplary plant-derived phenolics comprise syringyl phenolics, guaiacyl phenolics, and p-hydroxyphenyl phenolics. Exemplary syringyl phenolics include syringaldehyde, syringic acid, and S-diketone. Exemplary guaiacyl phenolics include vanillin, vanillic acid, and G-diketone. Exemplary hydroxyphenyl phenolics include p-coumaric acid, p-hydroxybenzaldehyde, and p-hydroxybenzoic acid.

The plant-derived phenolic can be derived and/or provided in the form of depolymerized lignin, such as chemically depolymerized lignin. Methods of depolymerizing lignin are well known in the art. See Pandey et al. 2010 (Pandey M P, Kim C S. Lignin Depolymerization and Conversion: A Review of Thermochemical Methods. Chemical & Engineering Technology, 2010, Vol. 34, Issue 1, pp. 3-145) and Wang et al. 2013 (Wang H, Tucker M, Ji Y. Recent Development in Chemical Depolymerization of Lignin: A Review. Journal of Applied Chemistry, 2013, Volume 2013, Article ID 838645).

The depolymerized lignin can be derived from pretreated lignocellulosic biomass. Methods of pretreating lignocellulosic biomass are well known in the art. See Kumar et al. 2017 (Kumar A K and Sharma S. Recent Updates on Different Methods of Pretreatment of Lignocellulosic Feedstocks: A Review. Bioresour. Bioprocess. (2017) 4:7); Kumar et al. 2009 (Kumar, P.; Barrett, D. M.; Delwiche, M. J.; Stroeve, P., Methods for Pretreatment of lignocellulosic Biomass for Efficient Hydrolysis and Biofuel Production. Industrial & Engineering Chemistry Research 2009, 48, (8), 3713-3729); Wang et al. 2013 (Wang H, Tucker M, Ji Y. Recent Development in Chemical Depolymerization of Lignin: A Review. (2013) Journal of Applied Chemistry. 2013:1-9), and Karlen et al. 2020 (Karlen S D, Fasahati P, Mazaheri M, Serate J, Smith R A, Sirobhushanam S, Chen M, Tymkhin V I, Cass C L, Liu S, Padmakshan D, Xie D, Zhang Y, McGee M A, Russell J D, Coon J J, Kaeppler H F, de Leon N, Maravelias C T, Runge T M, Kaeppler S M, Sedbrook J C, Ralph J. Assessing the viability of recovering hydroxycinnamic acids from lignocellulosic biorefinery alkaline pretreatment waste streams. ChemSusChem. 2020 Jan. 26). Examples include chipping, grinding, milling, steam pretreatment, ammonia fiber expansion (AFEX, also referred to as ammonia fiber explosion), ammonia recycle percolation (ARP), CO₂ explosion, steam explosion, ozonolysis, wet oxidation, acid hydrolysis, dilute-acid hydrolysis, alkaline hydrolysis, organosolv, ionic liquids, gamma-valerolactone, and pulsed electrical field treatment, among others.

The lignocellulosic biomass can be derived from any source, such as corn cobs, corn stover, cotton seed hairs, grasses, hardwood stems, leaves, newspaper, nut shells, paper, softwood stems, sorghum, switchgrass, waste papers from chemical pulps, wheat straw, wood, woody residues, mixed biomass species such as those produced by native prairie, and other sources.

In addition to the plant-derived phenolic, the medium in some versions also comprises a fermentable sugar. Non-limiting examples of suitable fermentable sugars include adonitol, arabinose, arabitol, ascorbic acid, chitin, cellubiose, dulcitol, erythrulose, fructose, fucose, galactose, glucose, gluconate, inositol, lactose, lactulose, lyxose, maltitol, maltose, maltotriose, mannitol, mannose, melezitose, melibiose, palatinose, pentaerythritol, raffinose, rhamnose, ribose, sorbitol, sorbose, starch, sucrose, trehalose, xylitol, xylose, and hydrates thereof, among others.

In some versions, the fermentable sugar may be replaced by other organic compounds that support growth of the recombinant microorganism. This includes but is not limited to the other organic compounds that are present in the deconstructed biomass fractions from the crops or plant species mentioned above.

A recitation herein of a microorganism “comprising” a mutation in or to a particular gene refers to a gene that would be present were it not for the mutation, e.g., the gene present in a corresponding microorganism. Thus, the recitation of a microorganism “comprising” a mutation in or to a particular gene encompasses a mutated form of the gene present in the microorganism, a partially deleted remnant of the gene present in the microorganism, a complete absence of the gene (e.g., as resulting from a complete deletion of the gene) in the microorganism, or other configurations.

The methods can further comprise isolating the 2-pyrone-4,6-dicarboxylic acid from the recombinant microorganism and/or the medium. Methods of isolating 2-pyrone-4,6-dicarboxylic acid from a medium are provided in the attached examples and otherwise known in the art.

The elements and method steps described herein can be used in any combination whether explicitly described or not.

All combinations of method steps as used herein can be performed in any order, unless otherwise specified or clearly implied to the contrary by the context in which the referenced combination is made.

As used herein, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise.

Numerical ranges as used herein are intended to include every number and subset of numbers contained within that range, whether specifically disclosed or not. Further, these numerical ranges should be construed as providing support for a claim directed to any number or subset of numbers in that range. For example, a disclosure of from 1 to 10 should be construed as supporting a range of from 2 to 8, from 3 to 7, from 5 to 6, from 1 to 9, from 3.6 to 4.6, from 3.5 to 9.9, and so forth.

All patents, patent publications, and peer-reviewed publications (i.e., “references”) cited herein are expressly incorporated by reference to the same extent as if each individual reference were specifically and individually indicated as being incorporated by reference. In case of conflict between the present disclosure and the incorporated references, the present disclosure controls. U.S. Pat. No. 10,144,938, US Pub. 2019/0048329, US Pub. 2016/0312257, and US Pub. 2020/0017891 are specifically incorporated herein by reference.

It is understood that the invention is not confined to the particular construction and arrangement of parts herein illustrated and described, but embraces such modified forms thereof as come within the scope of the claims.

EXAMPLES

Funneling Aromatic Products of Chemically Depolymerized Lignin into 2-Pyrone-4-6-Dicarboxylic Acid with Novosphingobium aromaticivorans

Summary

Lignin is an aromatic heteropolymer found in plant biomass. Depolymerization of lignin, either through biological or chemical means, invariably produces heterogenous mixtures of low molecular weight aromatic compounds. Microbes that can metabolize lignin-derived aromatics have evolved pathways that funnel these heterogeneous mixtures into a few common intermediates before opening the aromatic ring. In this work, we engineered a previously described recombinant form of Novosphingobium aromaticivorans DSM12444, via targeted gene deletions, to use its native funneling pathways to simultaneously convert plant-derived aromatic compounds containing syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) aromatic units into 2-pyrone-4,6-dicarboxylic acid (PDC), a potential polyester precursor. In batch cultures containing defined media, the engineered strain converted several of these depolymerization products, including S-diketone and G-diketone (non-natural compounds specifically produced by chemical depolymerization), into PDC with yields ranging from 22% to 100%. In batch cultures containing a heterogeneous mixture of aromatic monomers derived from chemical depolymerization of poplar lignin, 59% of the measured aromatic compounds were converted to PDC. Production of PDC from other lignin sources is also shown. Our results show that N. aromaticivorans is an exemplary microbial platform for funneling heterogeneous mixtures of lignin depolymerization products into PDC or other commodity chemicals.

Introduction

The impact of fossil carbon utilization on the global environment has encouraged the search for sustainable strategies to convert renewable resources into fuels and chemicals. Biorefining, the industrial activity of deriving fuels and chemicals from plant biomass in a sustainable and economically viable manner, is essential to reduce the proportion of fossil fuels that power the global economy. Plant biomass, the most abundant renewable organic resource on Earth, is primarily composed of sugars and phenolic compounds.^(3,4) While there are already established approaches to derive fuels from the sugar components of plant biomass,⁵ effective methods for biomass deconstruction to recover and valorize the phenolic components are only starting to emerge.^(6,7) One source of phenolic compounds is lignin, an alkyl-aromatic heteropolymer that is interlinked with cellulose and hemicellulose in plant cell walls and accounts for up to 30% of the total lignocellulosic biomass weight.⁸ There are other sources of phenolics in plant biomass, such as arabinofuranosides in grasses^(9,10) or lignin bound p-hydroxybenzoate in some hardwoods.¹¹ The present examples show bio-based production of valuable chemicals from the phenolic components of plant biomass.

The most abundant biomass-derived phenolics can be classified based on the number of methoxy groups attached to the main phenyl structure; these are syringyl (S; two methoxy groups), guaiacyl (G; one methoxy group), and p-hydroxyphenyl (H; no methoxy groups) units.¹² Several approaches have been recently described for biomass deconstruction and lignin depolymerization that result in recovery of S, G, and H aromatic units.⁶ However, the heterogeneity of the resulting mixtures presents a major challenge for conversion into commodity chemicals because of the low quantity of valuable marketable compounds in deconstructed lignin samples and the technical limitations for their separation or purification from other components.⁷

The present examples explore microbial strategies for the conversion of deconstructed lignin into commodity chemicals since microorganisms have evolved strategies to metabolize and gain energy from the degradation of a large variety of aromatics compounds.^(13,14) Such strategies could be harnessed for the valorization of aromatic mixtures if the metabolic pathways are routed towards production of desirable chemical products.¹⁵ In general, microbial transformation of aromatic compounds occurs by a combination of upper metabolic pathways, which convert multiple compounds into key aromatic intermediates¹³ in what has been called “biological funneling”,¹⁶ and a central aromatic pathway that breaks the aromaticity and renders metabolic products that enter central carbon metabolism.^(13,14) Biological funneling has been recently described for the conversion of plant-derived phenolics to aromatic compounds such as vanillin¹⁷ and benzoic acid,¹⁸ and to non-aromatic compounds, such as cis,cis-muconate,¹⁹ β-keto adipate,²⁰ muconolactone,²⁰ 2-pyrone-4,6-dicarboxylic acid (PDC),^(21,22) pyridine-2,4-dicarb oxylic acid,²³ and polyhydroxyalkanoates.¹⁶ Some of these approaches require extensive metabolic re-routing and introduction of foreign pathways,^(19,22) while others rely on a small number of mutations that redirect aromatic metabolism to the product of interest.^(17,18)

Here we show the impact of gene deletions in the central aromatic catabolic pathways of Novosphingobium aromaticivorans DSM12444, an organism known or predicted to degrade a wide variety aromatic compounds²⁴ and to break down interlinkages in lignin,²⁵ that allow it to funnel a large diversity of plant-derived phenolics into PDC, a potential bioplastic and epoxy adhesives precursor.²⁶ A complete genome sequence is available for this α-proteobacterium (GenBank NC_007794.1), and the organism is amenable to genetic and genomic techniques needed to test the role of individual genes in aromatic metabolism, and model, engineer, or improve its pathways.²⁵ Specifically, we show that by using a defined set of mutations, N. aromaticivorans can be engineered to simultaneously produce PDC from all three major types of plant-derived phenolic compounds (S, G, and H). In addition, we find that this organism can metabolize aromatics simultaneously with the use of other organic carbon sources (such as glucose or those found in deconstructed plant biomass), a feature that allows mutant strains to excrete compounds derived from the incomplete metabolism of the aromatics. We predict that implementing the defined set of mutations described herein will result in the same or similar effects in other microorganisms. The present examples represent a valuable advance in using bacteria to funnel aromatic compounds into defined single commodities and shows that N. aromaticivorans an exemplary microbial chassis for valorization of lignin and other plant-derived aromatics.

Materials and Methods

Bacterial Strains, Growth Media and Culturing Conditions

A variant of N. aromaticivorans DSM12444 (strain 12444Δ1879) that lacks the gene Saro_1879 (coding sequence, SEQ ID NO:25, protein sequence, SEQ ID NO:26) (putative sacB; SARO_RS09410 in the recently reannotated genome in NCBI)²⁵ was used as a parent strain to create the deletion mutant strains 12444ΔligI (lacks gene Saro_2819; SARO_RS14300), 12444ΔdesCD (lacks the genes Saro_2864 and Saro_2865; SARO_RS14525 and SARO_RS14530), 12444ΔligIΔdesCD (lacks genes Saro_2819, Saro_2864, and Saro_2865), and others described herein. All genetic modifications used a variant of the plasmid pk18mobsacB,³⁷ which contains sacB and a kanamycin resistance gene. A detailed procedure for constructing strains with gene deletions is contained elsewhere herein. All bacterial strains and plasmids used in the present examples are listed in Table 1. Primers used in the construction of the mutant strains are listed in Table 2.

TABLE 1 Bacterial strains and plasmids used in the examples. Strains Details Reference Novosphingobium aromaticivorans strains 12444Δ1879 DSM 12444 (WT) ΔSaro1879 (25) 12444ΔligI 12444Δ1879 ΔSaro2819 This study 12444ΔdesC/D 12444Δ1879 ΔSaro2864/5 This study 12444ΔligIΔdesC/D 12444Δ1879 ΔSaro2819 ΔSaro2864/5 This study Escherichia coli strains DH 5α F-Φ80lacZΔM15 Δ(lacZYA-argF) Bethesda U169 recA1 endA1 hsdR17 (rK−, Research mK+) phoA supE44λ-thi-1 gyrA96 Laboratories relA1 (44) S17-1 recA pro hsdR RP4-2-Tc::Mu- (45) Km::Tn7 Plasmids pK18mobsacB pMB1ori sacB kanR mobT oriT(RP4) (37) lacZα pK18msB/ pK18mobsacB containing genomic This study ΔSaro2819 regions flanking Saro2819 pK18msB/ pK18mobsacB containing genomic This study ΔSaro2864/5 regions flanking Saro2864/5

TABLE 2 Primers used in the examples. Name Sequence* SEQ ID NO Saro2819_Del-R 5′-GCGCCAATCCATACCACGGATTATGCGAATACTACT 27 CCATCCATCAGCTTG-3′ Saro2819- 5′-CGATTCATTAATGCAGCTGGCACGACAGGAGCGAAT 28 pK18_Amp-F GGCATGAGTTCACATTCAGC-3′ Saro2819_Del-F 5′-GCTGATGGATGGAGTAGTATTCGCATAATCCGTGGT 29 ATGGATTGGCGCATG-3′ Saro2819- 5′-GTTTCTGCGGACTGGCTTTCTAGATGTTCCTGCATG 30 pK18_Amp-R GTCTGGTCCTGTTCAAGCAG-3′ Saro2864-5Del R 5′-GGGTAGTCTGGATCATTCAGACTCGCATGGTGCCGA 31 G-3′ Saro2864-5- 5′-CGATTCATTAATGCAGCTGGCACGACAGCAGGTCGG 32 pK18_Amp_F CTTCAAGGAGGAAGTTCTG-3′ Saro2864-5_Del_F 5′-CCATGCGAGTCTGAATGATCCAGACTACCCGCCGTT 33 ATC-3′ Saro2864-5- 5′-GTTTCTGCGGACTGGCTTTCTAGATGTTCGACCACT 34 pK18_Amp_R ATGCAATGGAATGGAACCTGC-3′ Saro2865_Start- 5′-GGCATGCTCGGCACCATGCG-3′ 35 SNP_F Saro2865_Start- 5′-GCCGTCGACCGCGAGAGCTTG-3′ 36 SNP_R2 *Regions in bold match sequence in pK18msB.

Escherichia coli cultures were grown in LB media containing 50 μg mL⁻¹ kanamycin at 37° C. N. aromaticivorans cultures were grown in SISnc-V0 media supplemented with the indicated carbon source at 30° C. SISnc-V0 media is a modification of Sistrom's minimal media³⁸ in which succinate, L-glutamate, L-aspartate, and vitamins were omitted. For routine culture and storage, the growth media was supplemented with 1 g L⁻¹ glucose. For gene modifications, the growth media was supplemented with 1 g L⁻¹ glucose and 50 μg mL⁻¹ kanamycin, or 1 g L⁻¹ glucose and 10% sucrose.

N. aromaticivorans Growth Experiments

Cell cultures were grown overnight in SISnc-V0 media supplemented with 1 g L⁻¹ glucose, then diluted 1:1 with fresh SISnc-V0 containing 1 g L⁻¹ glucose and incubated for one hour. Then, 2 ml of the growing culture was spun for 5 min at 5000 rpm, and the cell pellets were resuspended into fresh SISnc-V0 media containing no added carbon source. The resuspended cells were diluted 1:100 into SISnc-V0 media supplemented with the indicated carbon source, then shaken at 200 rpm and 30° C. Cell growth was monitored by measuring cell density using a Klett-Summerson photoelectric colorimeter with a red filter. For N. aromaticivorans, 1 Klett unit (KU) is equal to ˜8×10⁶ cfu ml⁻¹.²⁵ Culture samples (1 mL) were collected at various time points, spun for 5 min at 5000 rpm and 4° C., and the supernatants were filtered through 0.22 μM nylon syringe tip filters (Fisher Scientific), then stored at −20° C. Each culture was grown at least three times and the data shown corresponds to the results obtained from a representative culture. Conversion efficiency of aromatics to product was calculated by dividing the total amount of product by the total amount of aromatic substrates consumed. Conversion efficiencies reported correspond to the average and standard deviation of the efficiencies calculated for all replicates.

Production of PDC in a Fed-Batch Bioreactor

A 250 ml bioreactor (Infors, model Multifors 2) containing 130 ml minimum media with 12 mM glucose was inoculated with 2 ml of N. aromaticivorans strain 12444ΔligIΔdesCD culture that had been pre-grown overnight with glucose. After 7.5 h of batch incubation, the bioreactor was intermittently fed media containing 226 mM vanillic acid, 34 mM vanillin, 550 mM glucose, 15 g L⁻¹ ammonium sulfate, and 5% (v/v) DMSO. Culture pH was controlled by the addition of 1 M KOH when needed, to maintain pH 7. Temperature was maintained at 30° C. and the stirrer speed between 250 and 320 rpm. Air was used to deliver oxygen at a flow rate of 1 L min⁻¹. During 50 hours of operation, a total of 29 ml of feed solution was added.

Analysis of Extracellular Metabolites

Metabolite identification was performed by gas chromatography-mass spectrometry (GC-MS) of filtered culture supernatants. Sample aliquots (150 μL) were combined with 70 μL of 1 mM m-coumaric acid in water (internal standard), acidified with HCl to pH<2, and ethyl acetate extracted (3×500 μL). The three ethyl acetate extractions were combined, dried under a stream of N₂ at 40° C., and derivatized by the addition of 150 μL of pyridine and 150 μL of N,O-bis(trimethylsilyl)trifluoro-acetamide with trimethylchlorosilane (99:1, w/w, Sigma) and incubated at 70° C. for 45 min. The derivatized samples were analyzed on an Agilent GC-MS (GC model 7890A, MS Model 5975C) equipped with a (5% phenyl)-methylpolysiloxane capillary column (Agilent model HP-5MS). The injection port temperature was held at 280° C. and the oven temperature program was held at 80° C. for 1 min, then ramped at 10° C. min⁻¹ to 220° C., held for 2 min, ramped at 20° C. min⁻¹ to 310° C., and held for 6 min. The MS used an electron impact (EI) ion source (70 eV) and a single quadrupole mass selection scanning at 2.5 Hz, from 50 to 650 m/z. The data was analyzed with Agilent MassHunter software suite, using m-coumaric acid as internal standard.

Quantitative analysis of glucose and formic acid were performed on an Agilent 1260 infinity HPLC equipped with a refractive index detector (HPLC-RID) (Agilent Technologies, Inc., Palo Alto, Calif.) and an Aminex HPX-87H with Cation-H guard column (BioRad, Inc. Hercules, Calif.). The mobile phase was 0.02 N sulfuric acid at a flow rate of 0.5 ml min⁻¹.

Quantitative analysis of aromatic compounds and PDC were performed on a Shimadzu triple quadrupole liquid chromatography mass spectrometer (LC-MS) (Nexera XR HPLC-8045 MS/MS). The mobile phase was a binary gradient consisting of solvent A (water) and solvent B (0.1% formic acid in a 2:1 mixture of acetonitrile and methanol, v/v). The stationary phase was a Phemonenex Kinetex F5 column (2.6 μm pore size, 2.1 mm ID, 150 mm length, P/N: H18-105937). All compounds were detected by multiple-reaction-monitoring (MRM) and quantified using the strongest MRM transition (Table 3).

TABLE 3 Multiple reaction module (MRM) conditions for HPLC-MS quantification of compounds used in the present examples. Compound MW (g/mol) Parent (—) m/z Transition 1 Transition 2 Transition 3 PDC 184.103 183 183 −> 139.05 183 −> 111 183 −> 94.95 CE11 CE14 CE12 Protocatechuic acid 154.12 153 153 −> 108.95 153 −> 107.95 153 −> 90.95 CE14 CE25 CE27 p-hydroxybenzoic acid 138.12 137 137 −> 93 137 −> 65 CE15 CE30 Vanillic acid 168.15 167 167 −> 123.05 167 −> 108 167 −> 152.05 CE15 CE21 CE18 p-hydroxybenzaldehyde 122.12 121.2 121.2 −> 92.05 121.2 −> 93,10 121.2 −> 41 CE26 CE22 CE49 Syringic acid 198.17 197 197 −> 121.05 197 −> 153.10 197 −> 182.10 CE18 CE15 CE15 Vanillin 152.15 151 151 −> 136 151 −> 92 151 −> 108 CE17 CE22 CE24 p-Coumaric acid 164.16 163 163 −> 119.05 163 −> 93 163 −> 116.95 CE15 CE31 CE33 Syringaldehyde 182.18 181 181 −> 166.10 181 −> 151 181 −> 123 CE16 CE22 CE28 Ferulic acid 194.19 193 193 −> 149 193 −> 134 193 −> 133 CE13 CE16 CE27 G-diketone 194.19 193 193 −> 178.10 193 −> 136 193 −> 107 CE20 CE21 CE31 S-diketone 224.21 223 223 −> 208.10 223 −> 193.10 223 −> 165.10 CE19 CE20 CE27 ¹H-NMR Analysis

Nuclear magnetic resonance (NMR) spectroscopy was performed on a Bruker Biospin (Billerica, Mass.) Avance 500 MHz spectrometer equipped with a 5 mm quadruple-resonance ¹H/³¹P/¹³C/¹⁵N QCI gradient cryoprobe with inverse geometry (proton coils closest to the sample). Samples were prepared as ˜1 mg in 600 μL acetone-d₆.

Gel Permeation Chromatography (GPC) Analysis

Analytical GPC was performed on a Shimadzu LC20 with a photodiode array detector (SPD-M20A). Separation was performed using a PSS PolarSil linear S column (7.8 mm×30 cm, 5 μm) at 35° C. The mobile phase was 5.2 mM sodium phosphate buffer at pH 8, pumped at 0.5 mL min⁻¹, 60 min run time. The molecular weight distribution was calibrated at λ=254 nm using PDC (184 g mol⁻¹, 23.55 min) and poly(styrene sulfonate) sodium salts, Mp (retention time): 976 kDa (13.20 min), 258 kDa (13.55 min), 65.4 kDa (14.78 min), 47 kDa (16.07 min), 9.74 kDa (17.96 min), 4.21 kDa (19.433 min), and 2.18 kDa (20.35 min) from the PSS-psskit (Polymer Standards Service-USA, Inc, Amherst, Mass., USA). Monomer standards were also ran to establish the lower threshold of the column and confirmed that some of them interact with the stationary phase in the alkaline-water mobile phase, these were: rosmarinic acid (360 g mol⁻¹, 21.49 min), ferulic acid (194 g mol⁻¹, 26.63 min), p-coumaric acid (164 g mol⁻¹, 24.96 min), vanillic acid (168 g mol⁻¹, 24.22 min), p-hydroxybenzoic acid (138 g mol⁻¹, 24.87 min), and guaiacol (124 g mol⁻¹, 39.82 min). Compounds eluting from 17.0-22.7 min correspond to oligomeric lignin, while compounds eluting after 22.7 min, correspond to dimeric and monomeric compounds. It should be noted that no M_(w) values were calculated for peaks detected after 22.7 min, as they were outside the calibration range of the GPC column. In the control samples there were strong monomer signals eluting after 26.0 min, especially a pair of signals at ˜30 min with an absorption band at 375 nm. Most of these monomer signals were not present, or were much weaker, in the inoculated samples after 78 hours of incubation.

Preparation of Media Containing Depolymerized Lignin Products

Lignin was isolated by acid precipitation from pretreatment liquor of poplar biomass that had been pretreated by the copper alkaline hydrogen peroxide method (AHP-Cu).³⁹⁻⁴¹ The lignin was depolymerized using an adaptation of the oxidative methods described previously.⁴ Depolymerization products were recovered by ethyl acetate extraction, followed by solvent evaporation. This material was re-dissolved in water while adjusting the pH to 7.0 to favor solubilization of aromatic compounds. Consistent with reported products of oxidative depolymerization,⁴ quantitative HPLC-MS analysis showed concentrations of 1 mM G-diketone, 0.35 mM S-diketone, 0.37 mM syringic acid, 0.12 mM syringaldehyde, 0.44 mM vanillic acid, 0.1 mM vanillin, and 0.93 mM p-hydroxybenzoic acid in the final aqueous solution. For experiments with N. aromaticivorans, aliquots of this solution (25 mL) were mixed with concentrated (5×) SISnc-V0 media containing 1 g L⁻¹ glucose (20 mL) and water (55 mL).

Chemicals

Syringic acid, syringaldehyde, ferulic acid, vanillic acid, vanillin, p-coumaric acid, p-hydroxybenzoic acid, p-hydroxybenzaldehyde, and protocatechuate were purchased from Sigma-Aldrich (St Louis, Mo.). G- and S-diketones were synthesized according to the methods described elsewhere herein. PDC was produced by culturing N. aromaticivorans 12444ΔligI in 1 L of SISnc-V0 media supplemented with 3 mM vanillic acid and 0.5 g L⁻¹ (2.8 mM) glucose, and purified following a simplified version of published methods,⁴² obtaining a >97% pure chemical standard for GC-MS and LC-MS quantifications. Specific details of these procedures are detailed elsewhere herein. The identity of PDC was confirmed by comparing the GC-MS spectrum of TMS derivatives and the ¹H-NMR spectrum with those reported previously.⁴³

Construction of Deletion Mutants of N. aromaticivorans

Construction of plasmids for deleting genes Saro_2819 or Saro_2864/5. Regions of N. aromaticivorans genomic DNA containing ˜1100 bp upstream and downstream of Saro_2819 or Saro_2864/5 were PCR amplified separately using the pairs of primers Saro2819_Del-R/Saro2819-pK18_Amp-F and Saro2819_Del-F/Saro2819-pK18_Amp-R for Saro_2819, and Saro2864-5_Del_R/Saro2864-5-pK18_Amp_F and Saro2864-5_Del_F/Saro2864-5 pK18_Amp_R for Saro_2864/5 (Table S2). The pairs of DNA amplified flanking regions for each gene were combined with linearized pK18msB using NEBuilder® HiFi DNA Assembly Master Mix (New England Biolabs, Ipswich, Mass.) to produce the plasmids pK18msB/ΔSaro2819 and pK18msB/ΔSaro2864/5, respectively. A 32 bp region of Saro_2865 (including the start codon) is predicted to overlap with Saro_2866. To prevent transcription of this region of Saro_2865, this putative start codon of Saro_2865 was mutated by replacing a T by a C at position 3088561 in the genome (in addition to deleting the sequence of Saro_2865 downstream of the Saro_2866 stop codon). To mutate the Saro_2865 start site, PCR was performed on plasmid pK18msB/ΔSaro2864/5 using the primers Saro2865_Start-SNP_F and Saro2865_Start-SNP_R2, which were previously phosphorylated with polynucleotide kinase from Promega (Madison, Wis.). The amplified product was circularized with T4 DNA ligase from New England Biolabs to obtain the circular plasmid pK18msB/ΔSaro2864/5. The plasmids were then transformed into NEB 5-alpha competent E. coli (New England Biolabs). The transformed E. coli cells were then cultured in LB media+kanamycin and the plasmids purified using a Qiagen® Plasmid Maxi Kit (Qiagen, Germany).

Deletion of genes Saro_2819 and Saro_2864/5. The purified plasmids were then transformed into competent E. coli S17-1 and subsequently mobilized into N. aromaticivorans strain 12444Δ1879 or 12444ΔligI cells via conjugation. Transconjugant cells of N. aromaticivorans (single cross overs) were isolated on SISnc-V0 plates containing 1 g/L glucose and 50 ug/mL kanamycin. To select for cells that eliminated the plasmid via a second instance of homologous recombination (double crossovers), single crossover cells were cultured on SISnc-V0 media containing 1 g/L glucose and 10% sucrose. Double crossover cells were isolated on SISnc-V0 plates containing 1 g/L glucose and 10% sucrose. PCR amplified regions of the target genes were sequenced to verify the deletions.

Purification of PDC

PDC was biologically produced by culturing Novosphingobium aromaticivorans strain 12444ΔligI in SISnc-V0 media supplemented with 3 mM vanillic acid and 3 mM glucose. Cells were grown to stationary phase and the culture media spun at 5000 RPM for 10 minutes and then filtered using a 500 ml Rapid-Flow bottle top filter with 0.2 μM SFCA membrane (Thermo Scientific). The filtrate (˜900 mL) was transferred to a large 2 L separatory funnel and prepared for extraction of the acidic PDC by dilution with 50 mL brine (saturated sodium chloride) and 20 mL concentrated hydrogen chloride. The acidified PDC was extracted with ethyl acetate (4×100 mL). The combined ethyl acetate fraction (˜400 mL) was extracted with 0.1 M sodium hydroxide (4×50 mL). The combined sodium hydroxide fraction was acidified with 2 M hydrogen chloride (20 mL) and brine (50 mL), then extracted with ethyl acetate (3×100 mL). The combined ethyl acetate fraction was dried using anhydrous sodium sulfate, filtered through a qualitative cellulose filter (VWR 28320-100), and the solvent removed on a rotatory evaporator giving 297 mg of PDC as a light orange solid. A TMS derivatized sample of the isolated PDC was characterized by GC-MS (method described in materials and methods section), which showed that PDC was the only peak, indicating a fairly high purity. The identity and purity of the PDC was confirmed by comparison of the ¹H NMR data to previously published values. The NMR and GC-MS spectra indicated the purity of PDC to be approximate 97%.

Steps in the Synthesis of S Diketone

Synthesis of 4-acetyl Syringaldehyde

To a 100 mL round bottom flask with stir bar were added syringaldehyde (3.296 g, 18.09 mmol), acetic anhydride (3.2 mL, 33.85 mmol), diisopropyl ethyl amine (1 mL, 5.74 mmol), potassium carbonate (793 mg, 5.74 mmol), and dichloromethane (50 mL). The solution was allowed to stir at room temperature. After 24 hours, the reaction was added to a separatory funnel, washed with saturated sodium bicarbonate (3×100 mL), and concentrated in vacuo to yield 4-acetyl syringaldehyde as an off-white solid (3.812 g, 17.00 mmol, 94% yield). ¹H NMR (400 MHz, Chloroform-d) δ 9.91 (s, 1H), 7.16 (s, 2H), 3.91 (s, 6H), 2.37 (s, 3H). See Figure S6 of Perez et al.⁴⁶ for the ¹H NMR spectra of synthesized 4-acetyl syringaldehyde.

Synthesis of 1-(4-acetoxy-3,4-dimethoxyphenyl)-1-propene

An oven dried, 100 mL round bottom flask with stir bar was charged with ethyltriphenylphosphonium bromide (7.0 g, 18.85 mmol), outfitted with a rubber septum, and the atmosphere within it purged with nitrogen. Freshly distilled THF (50 mL) was added via syringe and cooled to −78° C. While stirring, a solution of 2.0 M lithium diisopropyl amide (9.5 mL, 19 mmol) was added to generate ethenyltriphenylphosphonium bromide. While this solution stirred for 30 minutes, an oven dried, 250 mL round bottom flask with stir bar was charged with 4-acetyl-syringaldehyde (3.812 g, 17.0 mmol), sealed with a rubber septum, and purged with nitrogen. Freshly distilled THF (50 mL) was added via syringe and cooled to −78° C. Once the aldehyde was fully dissolved, the ethenyltriphenylphosphonium bromide solution was transferred by cannula and positive pressure to the 4-acetyl-syringaldehyde solution in a dropwise manner over the course of 45 minutes. Upon completion, the reaction was allowed to stir at −78° C. for an hour. The reaction was then brought to room temperature and stirred for two hours. The solution was quenched with saturated aqueous ammonium chloride and concentrated under reduced pressure. The remaining solution was diluted with water and extracted with ethyl acetate (3×100 mL). The organic layer was then evaporated leaving behind a pale yellow solid. The crude was purified by flash silica chromatography (5:1 hexanes/ethyl acetate). Fractions corresponding to the desired product were combined and evaporated, leaving behind 1-(4-acetoxy-3,4-dimethoxyphenyl)-1-propene as a white powder (1.2 g, 5.36 mmol, 32% yield, 1.08:1 cis/trans). ¹H NMR (400 MHz, Chloroform-d) δ 6.54 (s, 2H), 6.35 (dq, J=11.6, 1.9, 1H), 5.79 (dq, J=11.6, 7.2 Hz, 1H), 3.82 (s, 6H), 2.34 (s, 3H), 1.92 (dd, J=7.2, 1.9 Hz, 3H). See Figure S7 of Perez et al.⁴⁶ for the ¹H NMR spectra of synthesized 1-(4-acetoxy-3,4-dimethoxyphenyl)-1-propene.

Synthesis of 1-(4-acetoxy-3,4-dimethoxyphenyl)-1,2-propane dione

To a 100 mL round bottom flask with stir bar were added 1-(4-acetoxy-3,4-dimethoxyphenyl)-1 propene (720 mg, 3.05 mmol), dichloro(p-cymene)Ru(II) dimer (69.2 mg, 0.042 mmol), tetrabutylammonium iodide (336.4 mg, 0.91 mmol), tert-butyl hydroperoxide (70% solution in water, 3.6 mL), toluene (20 mL), acetonitrile (20 mL), and water (2.2 mL). The solution was allowed to stir at room temperature for 30 minutes then quenched with an excess of saturated aqueous sodium thiosulfate. The organic layer was isolated, concentrated in vacuo to a thick residue, and then purified by flash silica chromatography (4:1 hexanes/ethyl acetate). The resulting bright yellow fractions corresponding to the product were combined and evaporated to yield 1-(4-acetoxy-3,4-dimethoxyphenyl)-1,2-propane dione as a bright yellow solid (445 mg, 1.67 mmol, 55% yield). ¹H NMR (400 MHz, Chloroform-d) δ 7.33 (s, 2H), 3.88 (s, 6H), 2.53 (s, 3H), 2.36 (s, 3H). See Figure S8 of Perez et al.⁴⁶ for the ¹H NMR spectra of synthesized 1-(4-acetoxy-3,4-dimethoxyphenyl)-1,2-propane dione.

Synthesis of 1-(4-hydroxy-3,4-dimethoxyphenyl)-1,2-propane dione (S-diketone)

To a 250 mL round bottom flask were added 1-(4-acetoxy-3,4-dimethoxyphenyl)-1,2-propane dione (445 mg, 1.67 mmol), 3 M HCl (35 mL), and methanol (75 mL). The solution stirred at room temperature and reaction progress was monitored by TLC. Upon completion, the reaction was concentrated, diluted with saturated sodium bicarbonate, and washed with ethyl acetate. The aqueous layer was acidified with dilute ammonium chloride and extracted with ethyl acetate (3×50 mL). The resulting organic layer was concentrated and purified by flash silica chromatography (4:1 hexanes/ethyl acetate). The desired fractions were combined and evaporated to yield 1-(4-hydroxy-3,4-dimethoxyphenyl)-1,2-propane dione (S-diketone) as a bright yellow solid (259 mg, 1.16 mmol, 69% yield). ¹H NMR (400 MHz, Chloroform-d) δ 7.34 (s, 2H), 6.11 (s, 1H), 3.95 (s, 3H), 2.53 (s, 3H). See Figure S9 of Perez et al.⁴⁶ for the ¹H NMR spectra of synthesized 1-(4-hydroxy-3,4-dimethoxyphenyl)-1,2-propane dione (S-diketone). Steps in the Synthesis of G Diketone

Synthesis of Isoeugenyl Acetate

To a 100 mL round bottom flask with stir bar were added isoeugenol (2.6 mL, 17.10 mmol), acetic anhydride (3.00 mL, 31.73 mmol), diisopropyl ethyl amine (1 mL, 5.74 mmol), potassium carbonate (793 mg, 5.74 mmol), and dichloromethane (500 mL). The solution was allowed to stir at room temperature. After 24 hours, the reaction was added to a separatory funnel, washed with saturated sodium bicarbonate (3×100 mL), and concentrated in vacuo. The resulting off white powder was recrystallized from hot acetone to yield isoeugenyl acetate as white crystals (2.292 g, 11.11 mmol, 65% yield). ¹H NMR (400 MHz, Chloroform-d) δ 6.95 (d, J=8.1 Hz, 1H), 6.92 (d, J=1.8 Hz, 1H), 6.89 (dd, J=8.1, 1.9 Hz, 1H), 6.36 (dq, J=15.6, 1.7 Hz, 1H), 6.18 (dq, J=15.7, 6.6 Hz, 1H), 3.84 (s, 3H), 2.30 (s, 3H), 1.88 (dd, J=6.6, 1.6 Hz, 3H). See Figure S10 of Perez et al.⁴⁶ for the ¹H NMR spectra of synthesized isoeugenyl acetate.

Synthesis of 1-(4-acetoxy-3-methoxyphenyl)-1,2-propane dione

To a 250 mL round bottom flask with stir bar were added isoeugenyl acetate (2.060 g, 9.99 mmol), dichloro(p-cymene)Ru(II) dimer (69.2 mg, 0.11 mmol), tetrabutylammonium iodide (1.12 g, 3.03 mmol), tert-butyl hydroperoxide (70% solution in water, 10 mL), toluene (30 mL), acetonitrile (30 mL), and water (7 mL). The solution was allowed to stir at room temperature for 45 minutes then quenched with an excess of saturated aqueous sodium thiosulfate. The organic layer was isolated, concentrated in vacuo to a thick residue, and then purified by flash silica chromatography (4:1 hexanes/ethyl acetate). The resulting bright yellow fractions corresponding to the product were combined and evaporated to yield 1-(4-acetoxy-3-methoxyphenyl)-1,2-propane dione as a bright yellow solid (1.28 g, 5.42 mmol, 54% yield). ¹H NMR (400 MHz, Chloroform-d) δ 7.66 (d, J=1.9 Hz, 1H), 7.64 (dd, J=8.1, 1.9 Hz, 1H), 7.16 (d, J=8.2 Hz, 1H), 3.90 (s, 3H), 2.52 (s, 3H), 2.34 (s, 3H). See Figure S11 of Perez et al.⁴⁶ for the ¹H NMR spectra of synthesized 1-(4-acetoxy-3-methoxyphenyl)-1,2-propane dione.

Synthesis of 1-(4-hydroxy-3-methoxyphenyl)-1,2-propane dione (G-diketone)

To a 500 mL round bottom flask were added 1-(4-acetoxy-3-methoxyphenyl)-1,2-propane dione (1.00 g, 4.23 mmol), 3 M HCl (90 mL), and methanol (190 mL). The solution was stirred at room temperature and reaction progress was monitored by TLC. Upon completion, the reaction was concentrated, diluted with saturated sodium bicarbonate, and washed with ethyl acetate. The aqueous layer was acidified with dilute ammonium chloride and extracted with ethyl acetate (3×100 mL). The resulting organic layer was concentrated and purified by flash silica chromatography (4:1 hexanes/ethyl acetate). The desired fractions were combined and evaporated to yield 1-(4-hydroxy-3-methoxyphenyl)-1,2-propane dione as a bright yellow, viscous oil (526 mg, 2.71 mmol, 64% yield). ¹H NMR (400 MHz, Chloroform-d) δ 7.61 (dd, J=8.3, 1.9 Hz, 1H), 7.58 (d, J=1.9 Hz, 1H), 6.98 (d, J=8.3 Hz, 1H), 6.21 (s, 1H), 3.97 (s, 3H), 2.51 (s, 3H). See Figure S12 of Perez et al.⁴⁶ for the ¹H NMR spectra of synthesized 1-(4-hydroxy-3-methoxyphenyl)-1,2-propane dione (G-diketone). Results Model of Aromatic Metabolism By N. aromaticivorans DSM12444 and Justification of Experimental Approach

N. aromaticivorans DSM12444, a bacterium isolated from a polyaromatic hydrocarbon-contaminated sediment in the deep subsurface, aerobically utilizes a variety of aromatic compounds as sole carbon and energy sources for growth.²⁴ Based on its genome content, a recent analysis of N. aromaticivorans aromatic metabolism using a transposon library,² and the known metabolism of lignin-derived aromatics in the related α-proteobacterium Sphingobium sp. SYK-6,¹ we propose a model for the degradation pathways of plant-derived aromatic compounds in this organism (FIG. 1). Consistent with the predicted pathways in N. aromaticivorans and Sphingobium sp. SYK-6, we propose that G and H aromatic units are degraded via protocatechuic acid (FIG. 1), with ring opening by LigAB, a 4,5 extradiol dioxygenase that yields 4-carboxy-2-hydroxy-cis,cis-muconate-6-semialdehyde (CHMS). CHMS is then oxidized to PDC by the dehydrogenase LigC. LigI is predicted to hydrolyze PDC to produce 4-oxalomesaconate (OMA),²⁷ which is further transformed to the central carbon metabolites pyruvate and oxaloacetate (FIG. 1).

Dimethoxylated aromatics (S aromatics) are predicted to be degraded via a separate pathway, with demethylation of syringic acid to 3-methylgallate (3-MGA) carried out by the O-demethylase DesA (FIG. 1). In N. aromaticivorans, LigAB has been proposed to catalyze ring opening to produce a mixture of stereoisomers of 4-carboxy-2-hydroxy-6-methoxy-6-oxohexa-2,4-dienoate (CHMOD); a cis-trans isomerase, DesD, isomerizes one of the stereoisomers, and the methylesterase DesC completes demethylation of CHMOD to OMA.² Two other routes of 3-MGA degradation are proposed in Sphingobium sp. SYK-6, one requiring ring opening by the 3,4-dioxygenase DesZ and cyclization to PDC and another one requiring O-demethylation to gallate by LigM followed by ring opening by the dioxygenase DesB.¹ While LigM is present in N. aromaticivorans, homologues of DesZ and DesB are not encoded in its genome.² In addition, the LigAB of Sphingobium sp. SYK-6 has been shown to produce a combination of CHMOD and PDC when 3-MGA is the substrate,²⁸ and there are reports of slow abiotic transformation of CHMOD to PDC.²⁹ Therefore, in our model (FIG. 1), we hypothesize that the main enzymatic route of 3-MGA degradation in N. aromaticivorans is via CHMOD to OMA, but that PDC may also be a product of enzymatic or abiotic CHMOD transformation.

We used the above model to hypothesize which disruptions in the aromatic degradation pathways in N. aromaticivorans would lead to accumulation of specific pathway intermediates. We chose to focus on creating mutations that could lead to accumulation of PDC (FIG. 1), which is of particular interest since this dicarboxylic acid has been shown to be a suitable precursor for polyesters.³⁰ We hypothesized that a disruption of the proposed G and H degradation pathway via the deletion of the ligI gene (FIG. 1) would prevent PDC degradation and lead to its accumulation in cultures fed G and H aromatics as substrates. Furthermore, we predicted that this metabolic disruption would result in strains with limited ability to grow on G and H aromatics, since most of the carbon in these compounds would remain in the PDC molecule. If this latter prediction is correct, then the addition of another substrate would be needed to support growth of cells on G or H aromatics lacking a functional ligI gene. In addition, given the possibility of PDC production from CHMOD (FIG. 1), we also hypothesized that deleting the desCD genes would result in accumulation of upstream intermediates and redirection of metabolism via PDC (FIG. 1).

Below we describe how we tested these hypotheses and how the defined mutations lead to PDC accumulation from (i) G and H units, (ii) S, G, and H units, and (iii) aromatics that are present in depolymerized lignin.

Construction of a N. aromaticivorans Mutant that Accumulates PDC from G and H Aromatics

We constructed strain 12444ΔligI by deleting the ligI gene and cultured it initially in minimal media containing glucose since this gene was not predicted to be necessary for glucose metabolism. To test the role of this gene in metabolism of aromatic compounds, we attempted to grow strain 12444ΔligI on minimal media containing 3 mM vanillic acid or 3 mM p-hydroxybenzoic acid as representative of G and H aromatics, respectively. As expected, strain 12444ΔligI was unable to grow on either of these substrates as sole carbon sources (FIG. 2). When glucose was provided in addition to vanillic acid or p-hydroxybenzoic acid, strain 12444ΔligI was able to grow (FIG. 3 panels A and B), and, in both cases, glucose and the aromatic substrate were removed from the media, a small amount of protocatechuic acid transiently accumulated, and PDC accumulated as the final product of the transformations (FIG. 3 panels C and D). The PDC yield from vanillic acid and p-hydroxybenzoic acid by strain 12444ΔligI in these cultures were 81% (±17%) and 73% (±1.7%), respectively (Table 4).

TABLE 4 PDC yield from different aromatic compounds by N. aromaticivorans strains 12444ΔligI (p-coumaric acid, p-hydroxybenzaldehyde, p-hydroxybenzoic acid, ferulic acid, vanillin, and vanillic acid) and 12444ΔligIΔdesCD (G-diketone, syringaldehyde, syringic acid, and S-diketone) into PDC. Numbers in parenthesis represent one standard deviation of the average from 3 replicate cultures. Compound R Yield (%)

p-Coumaric acid

84 (±5.4) p-Hydroxybenzaldehyde

79 (±2.0) p-Hydroxybenzoic acid

73 (±1.7)

Ferulic acid

76 (±10.0) Vanillin

100 (±0.1) Vanillic acid

81 (±17.0) G-diketone

107 (±1.6)

Syringaldehyde

90 (±7.2) Syringic acid

66 (±13.2) S-diketone

22 (±0.7)

In theory, other G and H aromatics metabolized by N. aromaticivorans would also produce PDC when fed to strain 12444ΔligI (FIG. 1). We tested this prediction with the G aromatics vanillin and ferulic acid and the H aromatics p-hydroxybenzaldehyde and p-coumaric acid (FIG. 4 and Table 4). Cultures grown on minimum media with 3 mM vanillin plus 3 mM glucose showed transient accumulation of vanillic acid (FIG. 4 panel A), then a nearly stoichiometric accumulation of PDC. In the cultures grown with glucose and p-hydroxybenzaldehyde (FIG. 4 panel B), a transient accumulation of extracellular p-hydroxybenzoic acid and protocatechuic acid was observed, then accumulation of PDC with a 79% (±2%) yield (Table 4). Cultures grown on ferulic acid plus glucose showed a transient accumulation of vanillic acid and protocatechuic acid (FIG. 4 panel C), then accumulation of PDC with a 76% (±10%) yield (Table 4). Similarly, the cultures grown with p-coumaric acid and glucose transiently accumulated extracellularp-hydroxybenzoic and protocatechuic acids (FIG. 4 panel D), then accumulated PDC with an efficiency of 84% (±5.4%) (Table 4).

These results are consistent with transformation of G and H aromatics via the predicted pathway of FIG. 1. The observed PDC yields (Table 4) suggest that PDC is the main intermediate that accumulates, and that disruption of the ligI gene is sufficient to prevent PDC catabolism.

The inability of 12444ΔligI to metabolize PDC is not predicted to affect the degradation of aromatics containing S units, since the metabolism of these compounds would follow the 3-MGA, CHMOD, OMA pathway (FIG. 1). In agreement with this hypothesis, when strain 12444ΔligI was fed 3 mM syringic acid as the sole carbon source, growth of this mutant reached final cell densities similar to those of parent strain 12444Δ1879 and this aromatic was metabolized to a similar extent in both strains (FIG. 5). This observation confirms that LigI is not necessary for syringic acid degradation. However, these experiments also showed that PDC accumulates in the growth media in both cases, representing 28% (0.97 mM) and 26% (0.89 mM) of the initial concentration of syringic acid for strains 12444Δ1879 and 12444ΔligI, respectively.

Construction of an N. aromaticivorans Mutant that Accumulates PDC from S Aromatics

Dimethoxylated phenolics, such as syringic acid, are predicted to be degraded by N. aromaticivorans via the 3-MGA, CHMOD, OMA pathway (FIG. 1). Based on this prediction, we hypothesize that deleting the desCD genes would disrupt the degradation of S aromatics (FIG. 1), leading to the accumulation of the intermediate CHMOD. However, this mutation may not be sufficient to prevent growth of N. aromaticivorans on S aromatics because CHMOD may undergo abiotic or enzymatic transformation to PDC,²⁹ which could then be hydrolysed by LigI. Thus, to test these hypotheses, we constructed strain 12444ΔdesCD by deleting the desCD genes from strain 12444Δ1879.

Growth was not observed when strain 12444ΔdesCD was cultured in minimal media with 3 mM syringic acid as the sole carbon source (FIG. 6 panel A), indicating that either desC, desD or both genes are essential for growth on syringic acid, in agreement with observations reported previously.² To test the 12444ΔdesCD strain for a defect in S aromatic metabolism when growth was occurring, we inoculated the strain into media containing both 3 mM glucose and 3 mM syringic acid (FIG. 6 panel C). The 12444ΔdesCD strain grew, with consumption of both syringic acid and glucose, and with increased PDC accumulation compared to strain 12444Δ1879, converting 49% (±0.9%) of the syringic acid into PDC (versus 28% for 12444Δ1879; FIG. 5 panel C). This suggests that increased cyclization of CHMOD to PDC took place, although this observation is not sufficient to determine whether the reaction is abiotic or enzymatic. Growth of 12444ΔdesCD on vanillic acid as the only carbon source demonstrated that the disruption in desCD does not affect the catabolism of G units and does not lead to detectable PDC accumulation (FIG. 7).

Construction of an N. aromaticivorans Mutant that Accumulates PDC from S, G, and H Aromatics

Based on the observations with strains 12444ΔligI and 12444ΔdesCD, we hypothesized that a mutant missing ligI and desCD would be able to produce a higher yield of PDC from S aromatics. We generated this strain (12444ΔligIΔdesCD) and found that when it was cultured in minimal media with 3 mM syringic acid as the sole carbon source, it did not grow, as expected from previously presented data (FIG. 6 panel B). When glucose was added to the growth media, strain 12444ΔligIΔdesCD grew (FIG. 6 panel D), glucose and syringic acid were removed from the media, and PDC accumulated (FIG. 6 panel F). Indeed, the PDC yield of 12444ΔligIΔdesCD (66%±13%), was higher than that of 12444ΔdesCD (49%±0.9%) (Table 4).

PDC production from syringaldehyde by strain 12444ΔligIΔdesCD was also tested. When this strain was grown on 1 mM syringaldehyde plus 3 mM glucose (FIG. 4 panel E), syringaldehyde disappeared from the growth media, syringic acid was transiently detected, and PDC accumulated with a 90% (±7%) yield (Table 4).

The Fate of Unconverted Aromatic Carbon

Since PDC yields were typically less than 100%, it is possible that some aromatic compounds are degraded via alternative routes not blocked by the ΔligI and ΔdesCD mutations, and therefore, a fraction of aromatics may be still used as carbon and energy sources for growth in strain 12444ΔligIΔdesCD. To evaluate this hypothesis, we compared cell yields in 12444ΔligIΔdesCD cultures grown on either 3 mM glucose or 3 mM glucose plus 3 mM protocatechuic acid. The cultures grown on glucose reached a final density of 165 (±1) Klett units and no glucose or PDC was detected in the culture media (Table 5). The cultures receiving glucose plus protocatechuic acid reached a final cell density of 202 (±2) Klett units (Table 5). In these cultures, all glucose was consumed and 0.2 mM (±0.03) protocatechuic acid remained in the growth media (Table 5). The calculated yield of PDC based on the consumed protocatechuic acid was 85% (±1%) (Table 5). Since in both conditions the same amount of glucose was provided, the higher cell density observed in the cultures containing glucose plus protocatechuic acid can be explained by the use of a fraction of protocatechuic as a carbon and energy source for cell growth, presumably via a less efficient alternative pathway. The absence of PDC in the cultures containing only glucose shows that strain 12444ΔligIΔdesCD does not produce PDC from glucose.

TABLE 5 Comparison of cell densities and extracellular concentrations at stationary phase of N. aromaticivorans strain 12444ΔligIΔdesCD cultures grown on glucose or glucose plus protocatechuic acid. Data shown represents the average of 3 biological replicates. Error bars represent ± one standard deviation. Glucose + Glucose protocatechuic acid Maximum cell density (Klett) 165.3 (±0.58) 201.7 (±2.08)  Metabolites concentration immediately after inoculation Glucose (mM)  3.1 (±0.02) 3.1 (±0.04) Protocatechuic acid (mM) 0.0 2.9 (±0.02) PDC (mM) 0.0 0.0 Metabolites concentration at stationary phase Glucose (mM) 0.0 0.0 Protocatechuic acid (mM) 0.0 0.2 (±0.03) PDC (mM) 0.0 2.3 (±0.04) PDC yield (%) 0.0  85 (±1.10) Production of PDC from Chemically Depolymerized Lignin

Lignocellulosic biomass pretreatment and chemical depolymerization of lignin typically result in heterogeneous mixtures of aromatics with variable molar yields of monomers recovered.^(6,7) Based on the above results, a strain lacking both LigI and DesCD activity might also be able to simultaneously convert all three classes (S, G, and H) of plant-derived aromatics into PDC. To test the ability of strain 12444ΔligIΔdesCD to produce PDC simultaneously from multiple S, G, and H aromatic compounds, we cultured it in glucose-containing media supplemented with the products of depolymerized poplar lignin,⁴ which contained a mixture of S, G, and H aromatic compounds (FIG. 9). For comparison, strain 12444Δ1879 was cultured in the same media. In addition, a flask containing the same media without cells was incubated as an abiotic control. A large proportion of the aromatic compounds present in this type of depolymerized lignin are S and G type diketones⁴ and no information has been previously reported about the ability of bacteria to degrade them. Thus, in the experiments below we also tested for metabolism of the S and G diketones and their potential conversion into PDC.

In the abiotic control, none of the aromatic compounds were transformed after 77.5 h of incubation (FIG. 8 panel D, FIGS. 10A-10D, and FIG. 11). In the inoculated cultures, both strains grew, and, in both cases, all the major aromatic compounds (G-diketone, S-diketone, p-hydroxybenzoic acid, vanillin, vanillic acid, syringaldehyde, and syringic acid) disappeared from the growth media (FIG. 9 and FIG. 8 panels B and C). PDC only accumulated in the 12444ΔligIΔdesCD cultures, reaching a concentration of 0.49 mM (±0.02), which corresponds to a molar yield of 59% (±1.9%) assuming that all of the above aromatics were used as a source of this compound (FIG. 9).

Gel permeation chromatography (GPC) was performed to determine the presence of, and evaluate changes in, oligomeric lignin fragments found in these depolymerized lignin samples (FIGS. 10A-10D and 11). This analysis showed presence of compounds with a wide range of molecular weights (M_(w)), which we grouped in 2 ranges (see Materials and methods). Based on the analysis of standards, compounds eluting between 17.0 and 22.7 min corresponded to oligomeric lignin fragments, while compounds eluting after 22.7 min are dimeric and monomeric compounds. An abiotic control showed that during 78 hours of incubation there was an observable increase in low M_(w) oligomers, likely from reactive monomer condensation, that resulted in an average M_(w) decrease from 857 to 722 Da (FIG. 11). Both microbial cultures showed a decrease in the dimeric and monomeric compounds (signals eluting after 22.7 min) compared to the abiotic control sample. As with the sample before incubation, both microbial cultures showed the decrease in oligomer M_(w) attributed to reactive monomer condensation, but not as much as in the abiotic control (FIG. 11). Accumulation of PDC in experiments with 12444ΔligIΔdesCD was observable by a peak at 23.55 min (FIGS. 10A-10D), corresponding to that of the PDC standard, which was not observed in the abiotic control or the experiment with the parent strain 12444Δ1879.

While the above data suggest that 12444ΔligIΔdesCD is able to convert the G, S, and H units found in depolymerized lignin into PDC, the lack of stoichiometric conversion into PDC makes it difficult to assess how well each substrate is metabolized and converted into this product. To specifically test PDC production from the S and G aromatic diketones, we grew cultures of N. aromaticivorans strain 12444ΔligIΔdesCD on minimum media supplemented with chemically synthesized S-diketone plus glucose or G-diketone plus glucose (see elsewhere herein for aromatic diketone synthesis procedures). In the cultures containing S-diketone, 12444ΔligIΔdesCD grew, glucose and the aromatic diketone disappeared from the growth media, and PDC accumulated with a yield of 22.0% (±0.7%) (Table 4, FIG. 12 panels A and C). On the other hand, in the cultures supplemented with G-diketone (which contained small amounts of vanillic acid and vanillin as impurities from the synthesis method) both glucose and the aromatic substrates disappeared and PDC accumulated (FIG. 12 panels B and D), with a nearly stoichiometric yield (107%±1.6%, Table 4) for G-diketone (assuming a 100% yield from the vanillic acid and vanillin impurities). From this, we conclude that strain 12444ΔligIΔdesCD metabolizes these S and G diketones, using pathways that are also involved in degradation of the S, G and H aromatics normally found in lignin, and converts them into PDC, albeit at different efficiencies.

Production of PDC from Vanillic Acid and Vanillin in a Fed-Batch Reactor

To study the feasibility of PDC production by strain 12444ΔligIΔdesCD at titers higher than those observed in batch cultures, we cultured the mutant strain in a pH-controlled fed-batch reactor in which a concentrated solution containing vanillic acid, vanillin, and glucose was intermittently fed. In this experiment, a maximum concentration of 26.7 mM (4.9 g L⁻¹) of PDC was reached after 48 hours of incubation (FIG. 13), which represents a more than 8 times higher concentration than observed in the batch experiments reported here. As the reaction progressed, an accumulation of glucose, vanillic, and protocatechuic acid was observed.

Production of PDC from Various Lignocellulosic Biomass Preparations

The ability of the N. aromaticivorans 12444ΔligIΔdesCD strain to produce PDC from different lignocellulosic biomass preparations was tested.

The production of PDC from poplar and sorghum lignocellulosic biomass prepared using mild alkaline pretreatment⁴⁷ was tested with the N. aromaticivorans 12444ΔligIΔdesCD strain. High amounts of PDC from both of these lignocellulosic biomass preparations was produced (FIGS. 14A and 14B).

To determine the feasibility of producing PDC from lignin isolated from lignocellulosic biomass via γ-valerolactone (GVL) pretreatment and subsequently subjected to hydrogenolysis, production of PDC from isolated compounds found in such preparations was tested with the N. aromaticivorans 12444ΔligIΔdesCD strain. PDC was produced from many of these compounds (FIGS. 15A-15C), including methyl guaiacol (FIG. 15A), propyl guaiacol (FIG. 15A), dihydroconiferyl alcohol (FIG. 15A), methyl syringol (FIG. 15B), p-hydroxy benzoic acid methyl ester (FIG. 15B), dihydrop-hydroxy cinnamic acid methyl ester (FIG. 15B), dihydrosyringol alcohol (FIG. 15C), and dihydroferulic acid methyl ester (FIG. 15C).

In light of the high proportion of compounds capable of serving as PDC precursors, production of PDC from poplar (FIG. 16A), switchgrass (FIG. 16B), sorghum (FIG. 16C), and maple (FIG. 16D) lignin isolated from lignocellulosic biomass via γ-valerolactone (GVL) pretreatment and subjected to hydrogenolysis⁴⁸ was tested using the N. aromaticivorans 12444ΔligIΔdesCD strain. High amounts of PDC was produced in each case (FIGS. 16A-16D).

These results show that the N. aromaticivorans 12444ΔligIΔdesCD strain is capable of producing PDC from lignocellulosic biomass derived from various sources, pretreatments, and processing methods.

Additional Mutations to the N. aromaticivorans 12444ΔligIΔdesCD Strain

The effects of additional mutations to the N. aromaticivorans 12444ΔligIΔdesCD strain on PDC production were tested. The additional mutations included deletions of ligM (Saro_2861), desA (Saro_2404), vanA (Saro_1872), and combinations thereof. LigM (produced from ligM) and VanA (produced from vanA) are each vanillate/3-O-methylgallate O-demethylases. DesA (produced from desA) is a syringic acid O-demethylase. The parent 12444ΔligIΔdesCD strain and the variants thereof were tested for PDC production from vanillic acid (FIGS. 17A-17H) and syringic acid (FIGS. 18A-18H) as substrates.

Deletion of each ligM (FIG. 18B), and desA (FIG. 18C), individually in the 12444ΔligIΔdesCD strain increased PDC yields from syringic acid. Deletion of vanA (FIG. 18D) in the 12444ΔligIΔdesCD strain had no significant effect on PDC yield from syringic acid. Deletion of ligM in combination with vanA (FIG. 18F) in the 12444ΔligIΔdesCD strain increased the PDC yield from syringic acid to near stoichiometric conversion. Deletion of ligM in combination with desA (FIG. 18E), desA in combination with vanA (FIG. 18G), or ligM in combination with desA and vanA (FIG. 18H) ablated the production of PDC from syringic acid. Deletion of ligM (FIG. 17B), desA (FIG. 17C), or vanA (FIG. 17D) individually, or deletion of vanA in combination with ligM (FIG. 17F) or desA (FIG. 17G) in the 12444ΔligIΔdesCD strain had no significant effect on PDC production from vanillic acid. However, deletion of ligM in combination with desA (FIG. 17E) or deletion of ligM in combination with desA and vanA (FIG. 17H) in the 12444ΔligIΔdesCD strain decreased the production of PDC from vanillic acid.

The results outlined above suggest that DesA reacts with syringic acid as a substrate and likely also reacts with vanillic acid as a substrate, that LigM reacts with vanillic acid and 3-MGA as substrates and likely also reacts with syringic acid as a substrate, and that vanA may react with 3-MGA as a substrate. A revised model of the pathways leading to production of PDC from S units (syringic acid), G units (vanillic acid), and H units (p-hydroxybenzoic acid) based on the results outlined above is provided in FIG. 19.

Discussion

The economic and environmental viability of producing fuels and chemicals from lignocellulose is tightly connected to the efficiency of its utilization. New methods are needed to efficiently utilize the recalcitrant aromatic fractions, such as lignin.³¹ Multiple chemical approaches have shown promising results for breaking down the complex lignin polymer into small molecule aromatic units.^(6,7) However, the heterogeneous nature of the depolymerization products obtained pose challenges for further upgrading to valuable products.³² One successful strategy to address the chemical heterogeneity is to funnel the mixture of compounds through convergent aromatic biodegradation pathways into one valuable product by interruption and/or redirection of the metabolic flow to a pathway intermediate.^(19,22,23) These studies suggest that a mixed approach that integrates chemical and biological tools has the potential to be an effective strategy to maximize the yield of desired products from lignin transformation. Some of the major challenges in biological funneling are the transformation of unnatural products resulting from chemical depolymerization for which microbial metabolic capabilities are unknown, the maximization of target product yield while minimizing the accumulation of undesired intermediates or end products, and the identification of industrially useful target molecules that could most readily be produced from lignin components via known metabolic pathways.¹⁵

The present study addresses each of these issues using mutant strains of N. aromaticivorans DSM12444, a microbe naturally capable of degrading S, G, and H type aromatic compounds, as a well as lignin derived aromatic dimers.^(25,33) We chose N. aromaticivorans DSM12444 due to its known or predicted ability to grow in the presence of multiple aromatic compounds, its suitability for genetic analysis and modification, its ability to co-metabolize aromatics in the presence of other organic compounds (such as sugars, which are another plentiful product of plant biomass degradation), the lack of toxicity of PDC to this organism, and the potential to produce single valuable products using defined mutants.

The efficiency of carbon recovery in valuable compounds depends on factors such as the target product, the minimization of undesired metabolic byproducts, and number or amount of substrates being metabolized by the bacterium. Products derived from metabolic intermediates in the upper aromatic catabolic pathways of bacteria like N. aromaticivorans DSM12444 should yield higher carbon recovery than products derived from lower pathways, where more carbon may have already been lost during degradation. We selected PDC as the target product for this study because, in addition to its proven potential as a polyester precursor,²⁶ it is the earliest compound in which the degradation pathways for S, G, and H aromatic compounds were predicted to converge in defined N. aromaticivorans mutants (FIG. 1).

The observation of PDC accumulation when strain 12444Δ1879 was grown on syringic acid (28%; FIG. 5 panel C) was surprising, since we had predicted that the majority of the syringic acid would follow the 3-MGA, CHMOD, OMA pathway (FIG. 1) when the pathway was not altered by mutation. Furthermore, we had predicted that any PDC formed during syringic acid degradation in this strain would be oxidized by LigI to OMA (FIG. 1). The sequential increase in PDC yield in strains 12444ΔdesCD (49%; FIG. 6 panel E) and 12444ΔligIΔdesCD (66%; FIG. 6 panel F) confirms the participation of DesC, DesD, and LigI in the degradation of S type aromatics in N. aromaticivorans and suggests that a large fraction of the syringic acid is naturally channeled through PDC. Since PDC does not accumulate in 12444Δ1879 cultures grown on the products from chemically depolymerized lignin (FIG. 9 panel D) we offer two alternative hypotheses that would need to be tested in the future. First, it is possible that G or H substrates regulate expression of LigI in N. aromaticivorans. Thus, LigI would be poorly or not expressed when S type aromatics are the sole carbon source, allowing for some PDC accumulation by strain 12444Δ1879 grown on syringic acid. On the other hand, LigI would be expressed when 12444Δ1879 is grown on the mixtures of S, G, and H aromatics present in depolymerized lignin, preventing PDC accumulation. Alternatively, since it is not known whether CHMOD transformation to PDC is abiotic or enzymatic, it may be possible that CHMOD is secreted into the growth media where it undergoes spontaneous cyclization, resulting in extracellular PDC accumulation. Higher PDC yields by 12444ΔdesCD and 12444ΔligIΔdesCD could then be explained by increased CHMOD secretion when the aromatic degradation pathways are blocked.

We observe nearly stoichiometric conversion of vanillin and G-diketone into PDC, without extracellular accumulation of other aromatics. However, conversion of p-coumaric acid, p-hydroxybenzaldehyde, p-hydroxybenzoic acid, ferulic acid, vanillic acid, syringaldehyde, syringic acid, and S-diketone to PDC was found to have somewhat lower efficiencies (Table 4). The non-stoichiometric conversion of these aromatic compounds into PDC by N. aromaticivorans is not due to accumulation of intermediate metabolites such as syringic acid, vanillic acid, p-hydroxybenzoic acid and protocatechuic acid, since they only accumulated transiently. Instead, the lower conversion efficiencies could potentially be explained by the presence of alternative, less efficient, and poorly studied pathways for the degradation of those compounds. For instance, the N. aromaticivorans genome contains multiple genes annotated as aromatic ring cleavage dioxygenases for which specificity has not yet been established.³⁴ The presence of a catechol degradation pathway in N. aromaticivorans that uses 2,3-cleavage of the aromatic ring has been suggested as a possible alternative pathway for protocatechuic acid degradation.² Such alternative non-specific reaction of a catechol dioxygenase could explain the observed lower efficiencies in the transformation of some G and H aromatics to PDC. This hypothesis is supported by the increased cell density observed in cultures of strain 12444ΔligIΔdesCD grown in media containing glucose plus protocatechuic acid compared to cultures only fed glucose (Table 5). Another enzyme with low substrate specificity appears to be the O-demethylase LigM, included in our model as catalyzing the demethylation of vanillic acid (FIG. 1). In Sphingobium sp. SYK-6, LigM is also predicted to catalyze O-demethylation of 3-MGA to gallate,¹ which is then proposed to be oxidized to OMA by either LigAB, a dioxygenase with broad specificity (FIG. 1), or DesB, an enzyme not present in N. aromaticivorans. Although this route for degradation of S aromatics is not predicted to be important in N. aromaticivorans, ² LigM activity with 3-MGA and LigAB activity with gallate could contribute to lowering the efficiency of PDC formation from S aromatics by bypassing the blockage in S aromatic degradation intended with the desCD mutation. These consideration are solved with the experiments deleting the vanillate/3-O-methylgallate O-demethylases outlined above. Thus, future identification and analysis of additional pathways involved in aromatic metabolism by N. aromaticivorans DSM12444 could provide useful information for further increasing the yield of PDC or other target chemicals by preventing aromatic substrates from being degraded by alternative routes.

Fed-batch experiments in a pH-controlled bioreactor showed an increase of up to 8.7 times in PDC titers with respect to titers obtained in batch experiments. These results show a promising potential for production of PDC from aromatic compounds. However, in this experiment, a progressive accumulation of aromatic substrates and glucose was observed. Additional research will be necessary to optimize culture conditions.

The efficiency of lignin conversion to a desired product is also impacted by the nature of the aromatic compounds that result from chemical lignin depolymerization, which may be different from natural products of environmental lignin depolymerization. Therefore, the existence of microbial pathways to metabolize these products could be crucial to increase product recovery. For example, formic-acid-induced depolymerization of oxidized lignin produces a high proportion of aromatic diketones,⁴ compounds that have also been reported to be present in lignocellulose dilute acid hydrolysates.³⁵ Biological sources of these or structurally related compounds have not been reported, so it was previously unknown whether N. aromaticivorans DSM12444 could metabolize these products or convert them into PDC or other valuable materials. In this study, we found that N. aromaticivorans can convert both S- and G-type diketones into PDC, indicating that they are also degraded via the predicted aromatic degradation pathways (FIG. 1). However, the upper pathway enzymes that transform the diketones to known intermediates in the aromatic degradation pathways remain unknown.

Finally, chemically depolymerized lignin yields a variety of higher molecular weight lignin derived products in addition to monomeric units.⁴ Sphingomonad bacteria, such as N. aromaticivorans DSM12444, are known or predicted to be capable of breaking most of the linkages found between aromatic subunits in natural lignin in defined ways that yield predictable mono-aromatic products that can be further metabolized.^(1,36) N. aromaticivorans, specifically, is known to be capable of degrading model aromatic dimers containing β-aryl-ether bonds²⁵ and its genome contains homologs of genes that code for the degradation of other aromatic dimers in Sphingobium sp. SYK-6.¹ This is an unexplored, but potentially important aspect of employing N. aromaticivorans as a platform microbe for valorization of mixtures of low molecular weight aromatic compounds generated from chemical depolymerization of lignin.

Aspects of the present examples are found in Perez et al.⁴⁶, which is incorporated herein by reference in its entirety.

CONCLUSIONS

A path to produce valuable products from the abundant and renewable raw material lignin is to integrate chemical and biological strategies to chemically depolymerize lignin into heterogeneous mixtures of compounds that are then funneled into a single valuable product using microbial catalysts. An ideal microbial catalyst would be capable of simultaneously converting aromatic compounds containing S, G, and H structures, including non-natural compounds generated by chemical depolymerization, into a single compound with high efficiency.

Here, we focused on the microbial production of PDC from aromatic products known to be generated by chemical methods of lignin depolymerization and direct, base-catalyzed release of aromatics from whole biomass. PDC has been shown to have potential as a precursor for polyesters and there is growing interest in using microbes to generate it from lignin.^(21,22) However, the range of lignin-derived aromatic substrates that could be converted into PDC was limited.^(21,22) This study expanded the range. Future improvement in PDC yields would require identification of alternative pathways that may be contributing to aromatic degradation. The information and strategies developed here with N. aromaticivorans DSM12444 can be implemented in other microbes.

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What is claimed is:
 1. A recombinant microorganism comprising modifications with respect to a corresponding native microorganism not comprising the modifications, wherein the recombinant microorganism is a bacterium capable of producing 2-pyrone-4,6-dicarboxylic acid, wherein the recombinant microorganism is Novosphingobium aromaticivorans, wherein the modifications comprise: a first modification, wherein the first modification comprises a mutation of a native 2-pyrone-4,6-dicarboxylic acid (PDC) hydrolase gene present in the corresponding native microorganism, wherein the first modification reduces PDC hydrolase activity in the recombinant microorganism with respect to the corresponding native microorganism; a second modification, wherein the second modification comprises a mutation of a native 4-carboxy-2-hydroxy-6-methoxy-6-oxohexa-2,4-dienoate (CHMOD) cis-trans isomerase gene present in the corresponding native microorganism, wherein the second modification reduces CHMOD cis-trans isomerase activity in the recombinant microorganism with respect to the corresponding native microorganism; and a third modification, wherein the third modification comprises a mutation of a native 4-carboxy-2-hydroxy-6-methoxy-6-oxohexa-2,4-dienoate (CHMOD) methyl esterase gene present in the corresponding native microorganism, wherein the third modification reduces CHMOD methyl esterase activity in the recombinant microorganism with respect to the corresponding native microorganism.
 2. The recombinant microorganism of claim 1, wherein the modifications further comprise a fourth modification, wherein the fourth modification comprises a mutation of a native vanillate/3-O-methylgallate O-demethylase gene present in the corresponding native microorganism, wherein the fourth modification reduces vanillate/3-O-methylgallate O-demethylase activity in the recombinant microorganism with respect to the corresponding native microorganism.
 3. The recombinant microorganism of claim 2, wherein the mutation of the native vanillate/3-O-methylgallate O-demethylase gene comprises at least one of: a mutation of a native gene encoding LigM of Novosphingobium aromaticivorans; and a mutation of a native gene encoding VanA of Novosphingobium aromaticivorans.
 4. The recombinant microorganism of claim 1, wherein the recombinant microorganism exhibits enhanced production of 2-pyrone-4,6-dicarboxylic acid with respect to the corresponding native microorganism.
 5. A method for producing 2-pyrone-4,6-dicarboxylic acid comprising culturing the recombinant microorganism as recited in claim 1 in a medium comprising a plant-derived phenolic.
 6. The method of claim 5, wherein the plant-derived phenolic comprises a phenolic selected from the group consisting of a syringyl phenolic, a guaiacyl phenolic, and ap-hydroxyphenyl phenolic.
 7. The method of claim 5, wherein the medium comprises depolymerized lignin comprising the plant-derived phenolic.
 8. The method of claim 5, wherein the medium comprises chemically depolymerized lignin comprising the plant-derived phenolic.
 9. The method of claim 5, further comprising isolating the 2-pyrone-4,6-dicarboxylic acid from the medium and/or the recombinant microorganism.
 10. The recombinant microorganism of claim 2, wherein the mutation of the native vanillate/3-O-methylgallate O-demethylase gene comprises each of: a mutation of a native gene encoding LigM of Novosphingobium aromaticivorans; and a mutation of a native gene encoding VanA of Novosphingobium aromaticivorans. 